CTLA4-Ig (abatacept): a promising investigational drug for use in type 1 diabetes

ABSTRACT

Introduction

Type 1 diabetes (T1D) is an autoimmune disease that results from the destruction of insulin-producing beta cells in the pancreas; it leads to the under or nonproduction of insulin. T1D is associated with numerous life-threatening micro- and macro-vascular complications and early deaths, hence the development of preventative strategies is a priority for research.

Areas covered

The authors outline the drawbacks of available treatments for T1D and assess the three key strategies for prevention, including immunomodulatory therapies which hold the most potential. This article examines CTLA4-Ig and its efficacy and safety profiles. Finally, the pharmacokinetic parameters and pharmacodynamic markers of abatacept are shown in vivo and in clinical trials, guiding dosage regimen recommendations for future investigational studies.

Expert opinion

Immunomodulation is one of the promising strategies for decelerating the progression of beta-cell destruction after the onset of T1D. It holds the advantage of specific immune modulation without systemic general immunosuppression. Preclinical and clinical studies have yielded promising data on the use of CTLA4-Ig in T1D. Variations in response to CTLA4-Ig might be partially explained by the existence of multiple T1D subtypes with varying baseline innate inflammatory/regulatory bias and the rate of C-peptide decline.

KEYWORDS:

• Abatacept
• autoimmune disease
• beta cells
• CTLA4-Ig
• immunomodulation
• T-cell co-stimulation
• type 1 diabetes
• diabetes

1. Introduction

Insulin is a glucose reducing hormone that is released in the body by beta cells found in the islets of Langerhans located in the pancreas. Type 1 diabetes is an autoimmune disease that results from the destruction of these insulin-producing beta cells in the pancreas. The loss of beta cells results in little or no production of insulin which necessitates the administration of exogenous insulin throughout the patients’ life. Much about the causing factors of type 1 diabetes remain unknown; however, it is thought to involve genetic and environmental factors. Nevertheless, the prevalence of type 1 diabetes is markedly increasing worldwide. According to the International Diabetes Federation (IDF), the estimated number of children and adolescents (less than 20 years) with type 1 diabetes in 2019 was over 1.1 million and the number of new cases per year is 128,900 with the highest number of type 1 diabetes cases found in Europe [1]. Diabetes is associated with many life-threatening micro- and macro-vascular complications and early deaths; therefore, preventative strategies are warranted.

Our understanding of the pathogenesis of type 1 diabetes has increased significantly, particularly with regard to epidemiology, heterogeneity, pancreatic pathology, and ways of prediction of the disease [2]. The advancement of technology has improved the use of machines such as insulin pumps and glucose monitors, which help type 1 diabetes patients monitor the condition. The existing delivery systems for insulin all, except for two that were recently approved by FDA, suffer from the inability to regulate insulin without patient intervention. However, at present, despite extensive scientific efforts and investments, the current preventative measures or cures for type 1 diabetes are yet to be installed.

There are three current different strategies for the prevention of type 1 diabetes or prevention of the complete loss of beta-cell function after the onset of type 1 diabetes [3]. First strategy is to reduce exposure of genetically prone individuals early in life to environmental factors that can initiate the pathophysiological process that leads to type 1 diabetes. The second prevention strategy is to stop the gradual loss of pancreatic beta cells in individuals with pre-diabetes or individuals who test positive for islet autoantibodies. The third prevention strategy is to preserve residual insulin secretion by protecting the existing beta-cell mass after the onset of type 1 diabetes, since the majority of patients with new-onset type 1 diabetes have residual insulin secretions [4,5]. Currently, screening for type 1 diabetes using multiple autoantibodies is only recommended in the research settings; however, it should also be considered if the individual has first-degree family members with type 1 diabetes because more evidence indicates that the presence of such autoantibodies can be used as an early-stage predictor of clinical hyperglycemia and diabetes [6–9]. However, screening programs for type 1 diabetes are not routinely implemented and patients are often present with acute symptoms of diabetes as the first manifestations of the disease. In addition, to the best of our knowledge, no primary or secondary prevention trial resulted in preventing the progression to clinical type 1 diabetes in at-risk individuals. Therefore, the most practical strategy nowadays is to hinder the progression of beta-cell dysfunction during the early stages of the disease. This review will focus on the available agents used for the prevention of complete loss of beta-cell function to preserve residual insulin secretion after the onset of type 1 diabetes, particularly immunomodulatory agents such as the cytotoxic T lymphocyte-associated antigen 4-immunoglobulin (CTLA4-Ig) or abatacept.

2. Current treatments for type 1 diabetes

Complications associated with the poor treatment of type 1 diabetes or lack thereof, are diverse, debilitating and could lead to early death. These complications range from diabetic ketoacidosis to diseases related to cardiovascular, neuropathy, and retinopathy. Thus, a strict diet regimen and close management on a daily basis are crucial for achieving normal blood sugar levels in type 1 diabetes patients. To ensure this, type 1 diabetes patients are typically provided with a glucose monitor to check blood glucose levels multiple times daily. If glucose levels are too high, the patient is required to self-administer an insulin injection subcutaneously several times a day accordingly. Another method of treatment is an insulin pump, which is a wearable device that continuously provides insulin to the body through a catheter found under the skin. Through this device, the patient can easily modify the amount of insulin injected and change their time intervals [2]. Reports of increased hypoglycemic episodes after continuous use of insulin pumps are sparse and warrants further investigation [10–12]. Although all the above-mentioned insulin delivery systems have proven effective in the regulation of type 1 diabetes, most do not subside the need for patient intervention [2]. In the last two years, two attempts to create ‘artificial pancreas’ consisting of a hybrid closed loop system that monitors glucose and automatically adjusts the delivery of insulin based on the user’s glucose reading were successful and been approved by FDA [13–15]. Despite the effectiveness of continuous glucose monitoring in improving glycemic control in patients with type 1 diabetes, the evidence regarding the effect on reduction of hypoglycemic incidence is moderate and yet to be evaluated using defined and standardized reporting criteria [16]. Other forms of current type 1 diabetes treatment include the transplantation of the pancreas or the islet cells. However, despite ongoing trials, neither of these transplantations have been proven safe or lacking detrimental complications, such as immunosuppression and rejection [10].

Seeing that the current available treatments are associated with many drawbacks, the need for preventative therapies becomes ever more important. Here we show that immunomodulatory therapies, as a secondary or tertiary prevention strategy, hold the potential in slowing down the progression of beta-cell dysfunction early in the disease [17]. The targeted early stages of such strategy are the presymptomatic stage 1 and stage 2 of type 1 diabetes (secondary prevention), in which β-cell autoimmunity demonstrated by the detection of two or more islet autoantibodies is present together with normoglycemia and dysglycemia, respectively; and the symptomatic stage 3 of type 1 diabetes (tertiary prevention), in which hyperglycemia with clinical symptoms starts to appear [9,17]. Examples of immunomodulatory agents are CTLA4-Ig (abatacept), TNF-α antagonist (etanercept), anti-CD3 (teplizumab and otelixizumab) and anti-CD20 (rituximab). This review will focus on CTLA4-Ig as one of the most effective and safe options.

3. Immunomodulatory therapy

In type 1 diabetes, genetic predisposition plays a main role in the autoimmune destruction of beta cells. This is due to the defects in the immune regulation that result from these genetic variants. Type 1 diabetes patients have been found to have different immune regulatory capacities than healthy individuals, rendering the regulation of autoimmunity as an intervention method an important step in restoring immune imbalances [18]. Central to this approach is to understand the mechanism used by the immune system to destruct the pancreatic beta cells. Here, it is important to note that T-cells play a key role in the immunogenesis of type 1 diabetes. For the full activation of T-cells, two signals are required [19]. The first signal results from the interaction between the T-cell receptor (TCR) and an antigen peptide presented on a major histocompatibility complex (MHC) by an antigen-presenting cell (APC). The second and most crucial signal for the full activation of the T-cell results from the interaction between the TCR and specific ligands on the APC (Figure 1). These ligands are CD80 and CD86, which are found on the APC, and CD28 located on the surface of the T-cell. This interaction results in a strong co-stimulatory signal required for the full activation of the T-cell. Therefore, the inhibition of the second signal to block the stimulation of the T-cell was proposed as a therapeutic strategy for autoimmunity [20]. Unlike immunosuppression, this strategy interferes with the early stages of T-cell activation; therefore, it does not result in heavy general immunosuppression. Consequently, different immunomodulatory agents were tested in patients with new-onset type 1 diabetes in an attempt to arrest autoimmunity to preserve beta cells function [3,21]. C-peptide levels were used as a marker for endogenous insulin production by beta cells in these studies [22]. For the purpose of this paper, the discussion will be about cytotoxic T lymphocyte-associated antigen 4-immunoglobulin (CTLA4-Ig) as an intervention in new-onset type 1 diabetes as it holds the potential for preserving beta-cell function. Information on other potentially promising agents in this disease area is available for the development of a wider perspective [17,23,24].

Figure 1. Schematic representation of the signals involved in the full activation of the T-Cell. (a). Induction of the first and second signals as a result of the interaction between the ligands on the Antigen-Presenting Cell (APC) and the receptors on the T-Cell. Induction of first signal results from the interaction between an antigen present on the major histocompatibility complex (MHC) and the T-cell receptor (TCR), while the second signal is induced by the binding of CD80/CD86 on APC to CD28 on the T-cell. Full activation of the T-cell requires both signals to be activated. Also, T-cell activation can be inhibited with binding of CD80/CD86 on the APC to CTLA4 protein on the T-cell. (b). The effect of CTLA4-Ig (Abatacept) as a blocker of the second costimulatory signal required for full activation of the T-cell.

4. Cytotoxic T lymphocyte-associated antigen 4-immunoglobulin (CTLA4-Ig)

4.1. Pharmacological effects of CTLA4-Ig

CTLA4 protein, which was discovered in 1987 [25], has been shown to be related to CD28, the most prominent co-stimulatory receptor for naïve T-cells, by having the same specific sequence of amino acids that are critical for binding with CD80 and CD86 on the APC. CTLA4 protein has a higher affinity to CD80 and CD86 than CD28 [26]. The CTLA4 protein was then found to be a negative regulator of the CD28-mediated T-cell co-stimulation [27,28]. Thus, CTLA4-Ig or abatacept is used for selectively blocking CD28 co-stimulation which interferes with T-cell activation, proliferation, and survival.

The antagonism of CD28 pathway has shown to be effective in transplant settings and autoimmune diseases in preclinical models [20]. In vivo studies revealed that the simultaneous blockade of CD28 and CD40 pathways, which are important regulators of T-cell-dependent immune response, resulted in the long-term survival of allogeneic skin graft and inhibited the rejection of cardiac graft [29]. Lin, et al. have found that cardiac allografts survived for a long time (median 30 days) in mice treated with CTLA4-Ig at the time of transplantation. The survival was further prolonged with the addition of donor-specific transfusion to CTLA4-Ig [30]. CTLA4-Ig therapy was able to inhibit the rejection of human pancreatic islet transplants in mice [31]. In the same context, two studies have demonstrated a new strategy to lengthen the survival of transplanted islets by modifying CTLA4-Ig fusion gene in porcine-derived islet xenografts [32] or expressing the CTLA4-Ig fusion gene in dendritic cells, in diabetic mouse models [33]. Further studies showed that CTLA4-Ig can be more effective when combined with other strategies. For example, Pawlick et al. have shown that CTLA4-Ig, when combined with anti-NKG2D, which is an antagonist of the Natural Killer Group 2, member D (NKG2D) receptor expressed on NK and CD8⁺ T-cells, resulted in prolonging the islet allograft survival and improved glucose tolerance in mouse models with type 1 diabetes [34]. Another study has indicated that the additional expression of the co-inhibitor of T-cell immunity regulation, B7-H4, significantly prolonged allograft survival of islet transplant mice [35].

Altogether, the available evidence suggests that the blocking of T-cell co-stimulation by CTLA4-Ig is effective in prolonging the survival of islet transplants in vivo and associated with much lower side effects than general immunosuppression. Also, the addition of other agents can be considered for a synergistic effect in prolonging allograft survival, particularly islet allografts.

4.2. Efficacy of CTLA4-Ig

CTLA4-Ig is clinically available as abatacept, which was approved by Food and Drug Administration (FDA) in 2005 for the treatment of Rheumatoid Arthritis (RA) and in 2009 for the treatment of Juvenile Idiopathic Arthritis (JIA) under the marketed name ‘Orencia.’ A Phase II, multicenter, multinational, placebo-controlled, double-blind, dose-finding clinical trial was conducted to evaluate the safety and preliminary efficacy of two co-stimulatory blocking drugs: CTLA4-Ig (abatacept) and LEA29Y (belatacept) for patients with refractory RA [36]. The study showed that treatment with CTLA4-Ig and LEA29Y resulted in a dose response for the primary outcome to reduce signs and symptoms of RA on day 85 (assessed by the American College of Rheumatology 20% improvement criteria (ACR20)). In addition, the two medications were well tolerated with only a few side effects [36]. After this study, many large clinical trials investigating the safety and efficacy of CTLA4-Ig in RA have trailed which ended with the approval of this agent in 2005 [37]. Since CTLA4-Ig was proven effective in modulating autoimmunity in RA with a satisfactory safety profile, clinical studies have been undertaken to investigate the possibility of using this agent in other autoimmune conditions such as type 1 diabetes. Abatacept is also being studied for the treatment of other autoimmune diseases including Crohn’s, multiple sclerosis, lupus, and psoriasis.

The destruction of pancreatic beta cells is facilitated by T-cell activation. Therefore, CTLA4-Ig has been investigated for its effectiveness in preserving beta cells and preventing the complete loss of insulin production in an attempt to delay the progression of type 1 diabetes and its complications. In 2011, a multicenter, double-blind, phase II, randomized controlled trial was conducted on 112 patients with new-onset type 1 diabetes aged 6–45 years. Patients were administered abatacept solution of 10 mg/kg, max 1000 mg/dose (n = 77) or a matching placebo (n = 35) through IV infusion on days 1, 14, 28 and monthly thereafter totaling to 27 infusions over 2 years. The C-peptide mean concentration at baseline was 0.743$\pm$0.42 nmol/L in the abatacept group and 0.745$\pm$0.31 nmol/L in the placebo group. At 2 years of treatment, stimulated C-peptide concentration was 59% higher in the abatacept group compared to the placebo group (p = 0.0029) (2-h AUC for C-peptide of 0.378 vs. 0.238 nmol/L, respectively; p = 0.0014). However, the analysis of mixed-effects model showed that the reduction in beta-cell functioning in the abatacept group is parallel to that of the placebo group. In general, the study showed that abatacept was capable of delaying the reduction of beta-cell function by 9.6 months [38]. Then, follow-ups were conducted with the patients for 1 year after cessation of treatment to determine if the treatment effect was persistent. The mean simulated C-peptide 2-h AUC was 0.217 nmol/L in the abatacept group vs. 0.141 nmol/L in the placebo group (p = 0.046). Abatacept was also able to slow the decline in beta-cell function by 9.5 months after 1 year of discontinuation of the treatment [39]. A more recent observational study has evaluated insulin sensitivity before and after treatment with abatacept in patients with RA. Insulin sensitivity was evaluated by calculating Matsuda Insulin Stimulation Index (ISI); abatacept significantly improved ISI after 6 months of treatment (p = 0.003) [40]. Altogether, abatacept was able to slow the reduction of beta-cell function early in the development of the disease (within 3 months of diabetes), and the effect was continued 1 year after the cessation of treatment but without much improvement. This might be explained by the aggressive autoimmune response at the onset of the disease which can be targeted with the co-stimulation blockade of T-cell activation by abatacept. These efforts lead in May 2013 that orphan drug designation in the U.S. by Orban Biotech LLC (Massachusetts, USA) was assigned to abatacept (Orencia) for the treatment of type 1diabetes mellitus patients with residual beta-cell function, yet not FDA-approved for orphan indication [41].

Since its launch in the U.S. by Bristol-Myers Squibb in 2005, Abatacept is available as an intravenous formulation. A subcutaneous formulation for the treatment of RA was granted a positive opinion in July 2012 and in 2013, this subcutaneous injection formulation of the product was approved by Bristol-Myers Squibb and an alliance company, Ono, for the treatment of rheumatoid arthritis in Japan. In 2016, a subcutaneous autoinjector formulation was approved in Japan. Prefilled subcutaneous injections are available in the U.S. for the treatment of JIA since 2017. In the same year, a new indication for adult psoriatic arthritis was added to the prescribing information leaflet of the product [42].

4.3. Safety and adverse events of CTLA4-Ig

While immunoregulatory therapies hold great promise for the treatment of type 1 diabetes, their inadequacy and serious toxicity have limited research efforts to find a lifelong immunosuppression approach [43,44]. However, CTLA4-Ig inhibits naïve T-cell activation; therefore, it selectively inhibits T-cell response to specific antigens instead of broad immunosuppression. The safety of abatacept was studied in many clinical trials and post-marketing surveillance studies (Table 1). Most of the trials evaluated the safety of subcutaneous (SC) and intravenous (IV) abatacept in patients with RA, with one study evaluating the safety of abatacept in type 1 diabetes patients. In type 1 diabetes patients, there was no difference in adverse effects between abatacept and placebo [38]. The reported infusion-related adverse events were of low frequency and clinically insignificant. In general, long-term abatacept, either IV or SC, was well tolerated with infections such as upper respiratory tract infections, nasopharyngitis, bronchitis, sinusitis, and urinary tract infections as the most common side effects in patients with RA [45,46]. Malignancies, infections, and autoimmune events were of a concern since abatacept modulates the immune function. However, large post-surveillance studies demonstrated that abatacept was not associated with the significant increase in malignancies, serious infections, and autoimmune events [45,46]. Abatacept also demonstrated low immunogenicity rates that occurred transiently [47,48]. It was demonstrated that immunogenicity is similar after subcutaneous and intravenous abatacept or after monotherapy and combination therapy with methotrexate [47,49], and it was low or increased transiently when switching between the two routes of administration [50]. The rate of injection-site reactions including erythema, hematoma, pain, and pruritus was only 3.5% [44]. One limitation for abatacept clinical use is the restriction of using vaccines within 3 months of treatment, which becomes much important in the pediatric population [38]. In addition, episodes of hypoglycemia were reported, some of which were severe [38].

Table 1. Studies evaluated the safety of CTLA4-Ig (abatacept).

Study (Author, year of publication, sample size and reference)	Study Design	Population	Drug, Route and Dose	Duration	Safety Results
1. Weinblatt, et al., 2013 (n = 7322)*[45]	Integrated analysis of safety data from 8 clinical placebo-controlled trials	Moderate to severe RA patients	- Abatacept, IV, 2 or 10 mg/kg/month - In addition to concomitant medications for RA (e.g. MTX, etanercept, nonbiologic DMARD, DMARD, biologic therapy)	- Data represent up to 8 years of treatment		Short-term period (≤ 12 months)				Cumulative period† (short-term plus long-term extensions) (n=4149)
						Abatacept (n=3173)		Placebo (n=1099)
						n (%); IR (95% CI)		n (%); IR (95% CI)		n (%); IR (95% CI)
					Total AE	399 (12.6);		136 (12.4);		1373 (33.1);
						18.10 (16.37, 19.97)		16.90 (14.18, 19.99)		14.61 (13.85, 15.41)
					Deaths	12 (0.4);		7 (0.6);		73 (1.8);
						0.51 (0.27, 0.90)		0.82 (0.33, 1.69)		0.60 (0.47, 0.76)
					Infections	1559 (49.1); 98.00 (93.20, 102.99)		537 (48.9); 92.46 (84.81, 100.62)		2998 (72.3); 75.68 (73.00, 78.44)
					Serious infections	85 (2.7); 3.68 (2.94, 4.55)		22 (2.0); 2.60 (1.63, 3.94)		332 (8.0); 2.87 (2.57, 3.19)
					Total malignancies	17 (0.5); 0.73 (0.42, 1.17)		5 (0.5); 0.59 (0.19, 1.37)		88 (2.1); 0.73 (0.58, 0.89)
2. Lahaye, et al., 2016 (n = 1017)[51]	Multicenter, prospective, cohort study	Elderly patients with RA	- Abatacept infusion (no available dose) - Majority of patients were receiving DMARDs	- 5 years of prospective follow up - follow up for 2 years		<50 years (n = 293)	50-64 years (n = 406)	65-74 years (n = 215)	≥75 years (n = 103)	p-value
						n (per 100 patient-years)
					Severe infections	18 (1.73)	70 (4.65)	45 (5.90)	30 (10.38)	<0.001
					Deaths	2 (0.19)	16 (1.06)	16 (2.09)	25 (8.65)	<0.001
					Cancer	2 (0.19)	22 (1.46)	13 (1.7)	11 (3.81)	<0.001
3. Iwahashi, et al., 2014 (n = 118)[48]	Double-blind, double-dummy, multicenter, phase II/III study	Japanese patients with RA	- Abatacept, SC, 125 mg weekly with IV loading 10 mg/kg on Day 1 - Abatapect, IV, 10 mg/kg/month - Both groups receiving background MTX	- 24 weeks of study period + 8 weeks after abatacept last dose (results include 32 weeks)			SC Abatacept (n=59)		IV Abatacept (n=59)
						n (%)			n (%)
					AEs	45 (76.3)			49 (83.1)
					Local ISR	0			0
					Serious AEs	4 (6.8)			3 (5.1)
					Deaths	0			0
					Infections and infestations	20 (33.9)			29 (49.2)
					Malignancies	1 (1.7)			1 (1.7)
					Autoimmune disorders	0			0
4. Mease, et al., 2017 (n = 424)‡[52]	Multicenter, placebo-controlled, Phase III trial	Psoriatic arthritis patients	- Abatacept, SC, 125 mg weekly	- Double-blind period: 24 weeks - Cumulative period: 52 weeks		Double-blind period				Cumulative period (n=398)
						Abatacept (n=213)		Placebo (n=211)
						n (%)		n (%)		n (%)
					AEs	116 (54.5)		112 (53.1)		273 (68.6)
					Serious AEs	6 (2.8)		9 (4.3)		34 (8.5)
					Local ISRs	1 (0.5)		1 (0.5)		5 (1.3)
					Deaths	0		0		0
					Infections	57 (26.8)		63 (29.9)		162 (40.7)
					Autoimmune events	0		0		1 (0.3)
					Malignancies	0		2 (0.9)		4 (1.0)
5. Westhovens, et al., 2009 (n = 509)[53]	Double-blind, randomized, multinational, placebo-controlled, phase IIIb trial	Patients with new onset RA and poor prognosis	- Abatacept, IV infusion, ~10 mg/kg (according to weight range), on days 1, 15 and 29, and then monthly - Both groups receiving background MTX	- 1 year		Abatacept + MTX (n=256)			Placebo + MTX (n=253)
						n (%)			n (%)
					AEs	217 (84.8)			211 (83.4)
					Serious AEs	20 (7.8)			20 (7.9)
					Infections	132 (51.6)			139 (54.9)
					Serious infections	5 (2.0)			5 (2.0)
					Autoimmune events	6 (2.3)			5 (2.0)
					Malignancies	1 (0.4)			0 (0.0)
					Death	2 (0.8)			4 (1.6)
6. Orban, et al., 2011 (n = 112)[38]	Double-blind, randomized, multicenter, placebo- controlled, phase III trial	Recent onset TID patients	- Abatacept, IV infusion, 10 mg/kg (maximum 1000 mg per dose), on days 1, 14, 28, and monthly thereafter	- 2 years		Abatacept (n=77)			Placebo (n=35)
						n (%)			n (%)
					Infection	32 (41.6)			15 (42.9)
					Hypoglycemia	3 (3.9)			1 (2.9)
					Allergy/immunology	2 (2.6)			0
					Secondary malignancy	1 (1.3)			0
					Death (accidental)	1 (1.3)			0
7. Moreland, et al., 2002 (n = 214)[36]	Double-blind, randomized, multinational, multicenter, placebo-controlled, pilot, dose-finding, clinical trial	RA patients	- Abatacept, IV infusion: 0.5, 2, or 10 mg/kg on days 1, 15, 29, and 57 - Belatacept, IV infusion: 0.5, 2, or 10 mg/kg on days 1, 15, 29, and 57	- 85 days		Abatacept (n=90)		Belatacept (n=92)		Placebo (n=32)
						n (%)		n (%)		n (%)
					Total AEs	73 (81.1)		76 (82.6)		24 (75.0)
					Serious AEs	4 (4.4)		4 (4.3)		4 (12.5)
8. Nash, et al., 2013 (n = 100)[47]	Open-label, parallel-arm, international, multicenter, phase III study	RA patients	- Abatacept, SC, 125 mg weekly with or without MTX	- Short-term period: 4 months - Long-term period: up to 20 months		Short-term period				Long-term period (n=90)
						Abatacept + MTX (n=51)		Abatacept only (n=49)
						n (%)		n (%)		n (%)
					AEs	37 (72.5)		32 (65.3)		78 (86.7)
					Serious AEs	2 (3.9)		3 (6.1)		13 (14.4)
					Infections and infestations	18 (35.3)		14 (28.6)		54 (60.0)
					Local ISR	3 (5.9)		4 (8.2)		0
					Deaths	0		0		0
					Malignancies	0		0		1 (1.1)
					Autoimmune events	0		0		1 (1.1)
9. Alten, et al., 2014 (n = 1,879)§ [46]	Integrated analysis of data from double-blind, open-label phases of 5 clinical trials	RA patients	Abatacept, SC Dosages: - Weight-tiered (75–200 mg) or fixed (125 mg) weekly. - 125 mg/week with prior IV abatacept loading dose (10 mg/kg) - 125 mg/week, without IV abatacept loading dose - 125 mg/week, patients switched from long-term IV abatacept - some patients received background MTX	- Data represent >4 years of treatment		SC Abatacept (n=1879)			IV Abatacept (n=4149)||
						n (%); IR (95% CI)¶			n (%); IR (95% CI)
					Overall serious AEs	384 (20.4); 9.97 (9.02–11.02)			1,373 (33.1); 14.61 (13.85–15.41)
					Infections	1,176 (62.6); 50.52 (47.72–53.50)			2,997 (72.3); 75.65 (72.96–78.40)
					Serious infections	74 (3.9); 1.79 (1.42–2.24)			332 (0.8); 2.87 (2.57–3.19)
					Deaths	25 (1.3); 0.59 (0.40–0.88)			73 (1.8); 0.60 (0.47–0.76)
					Malignancies	30 (1.6); 0.71 (0.50–1.02)			88 (2.1); 0.73 (0.58–0.89)
					Autoimmune events	57 (3.0); 1.37 (1.06–1.78)			232 (5.6); 1.99 (1.74–2.26)

Data in this table are presented as n (%). Data of studies 1 and 9 are presented also as IR (95% CI) as they are large safety analysis studies. IR: incidence rate per 100 patient-years, CI: confidence interval, AEs: adverse events, DMARD: disease-modifying antirheumatic drug, ISR: injection-site reaction, SC: subcutaneous, IV: intravenous, MTX: methotrexate, T1D: type 1 diabetes,

*: Total number of patients received IV abatacept in both short-term and cumulative periods.

†: Patients treated with abatacept for: <3 months are 3.4%; 3 to <12 months are 26.2%; 12 to <24 months are 13.1%; 24 to <36 months are 10.7%; 36 to <48 months are 7.3%; 48 to <60 months are 11.4%; and ≥60 months are 28.0%.

‡: This number represents randomized patients to abatacept or placebo.

§: Number of patients received SC abatacept in the trials included in this safety analysis.

||: Number of patients received IV abatacept in the cumulative period in study 1 in this table.

¶: IR of all AEs did not increase over time.

4.4. Pharmacokinetics (PK) and pharmacodynamics (PD) of CTLA4-Ig

The PK parameters and PD markers of SC and IV abatacept were evaluated and characterized in in vivo studies in mice, rats, and monkeys. In addition, PK/PD parameters of this agent were also studied in clinical trials on healthy volunteers and patients with RA (Table 2). In most studies, serum concentration of CTLA4-Ig was determined using enzyme-linked immunosorbent assay (ELISA). The mean values for selected PK parameters of abatacept are summarized in Table 3. The pharmacokinetics of abatacept in children are generally similar to those in adults, taking into account the effect of body weight as a cofounder [54]. The PK of abatacept was described in all studies by a two-compartment model, as expected with large therapeutic proteins, with a first-order absorption process after SC administration. Bioavailability after SC administration ranged between 60% and 110% in the different species studied, with bioavailability decreasing as doses were increased. This is also in agreement with the wide bioavailability range observed after SC administration of other therapeutic proteins. Immunogenic responses to the humanized nature of abatacept may have contributed to its lower SC bioavailability in different animal species compared to that in clinical trials. Its clearance and volume of distribution in the central compartment were found to increase almost linearly with baseline body weight, which is not expected with monoclonal antibodies, so clearance is greater in patients with higher body weight. This may partially be explained by the fact that abatacept elimination occurs by reticuloendothelial catabolism which is believed to be allometrically scalable by body weight. Abatacept is distributed to extracellular fluid rendering it allometrically scalable by body weight also. Clearance values range observed in humans were narrow, 14.4 to 21.7 mL/h. A dose proportional increase in C_max and AUC parameters was observed after single or multiple doses of abatacept in different animal species studied. Abatacept has a relatively long half-life ranging 18.8–129 h in mice, 64.3–170 h in rats, 90–160 h in monkeys, and around 116 h in humans. After multiple doses of abatacept, achieving steady state is expected to take weeks.

Table 2. Studies evaluated the PK/PD of CTLA4-Ig (abatacept).

Study	Study Design/Subjcts	Analysis of Abatacept	Study Intervention	PK Parameters
1- Iwahashi, et al., 2014 [48]	Double-blind, double-dummy randomized, phase II/III study/RA patients	Using electrochemiluminescence immunoassay (Meso-Scale Discovery, Rockville, MD, USA)	- SC abatacept, 125 mg weekly + IV loading of 10 mg/kg on day 1 -IV abatacept, 10 mg/kg monthly - both receiving background MTX - Duration: 169 days	PK parameters of abatacept in Japanese RA patients with inadequate response to MTX
						SC Abatacept (n = 30)		IV Abatacept (n = 29)
				Parameter		Mean		Mean
				Cmax (μg/mL)		42.6		277.4
				Cmin (μg/mL)		36.0		16.7
				AUCtau (μg.h/mL)		5,889.1		33,118.5
				Absolute F (%)		78.4		NA
2- Lon, et al., 2013 [55]	Modeling PK/PD study/Rats with collagen-induced arthritis	Using Human sCTLA-4 Platinum Enzyme-linked immunosorbent assay kit (eBioscience, Inc., San Diego, CA, USA)	- IV abatacept, single dose 10 mg/kg (n = 5) - SC abatacept, single dose 20 mg/kg (n = 6) - SC abatacept, multiple dose (one 20 mg/kg, and 4 additional 10 mg/kg doses every 2 days) (n = 6) - vehicle control (n = 6)	Population PK/PD parameter estimates of abatacept in rats with induced arthritis
				Parameter			Estimate Mean (%RSE)
				Cl (mL/day/kg)			21.8 (6.70)
				ClD (mL/day/kg)			27.5 (10.3)
				V1 (mL/kg)			69.5 (21.6)
				V2 (mL/kg)			61.9 (9.00)
				F (%), after subcutaneous			59.2 (9.20)
				$\tau$ (day), input period for SC dose			2.67 (7.80)
				Imax, maximum inhibition on paw edema			0.161 (64.0)
				IC50 (µmol/L)			0.731 (131.7)
3- Hasegawa, et al., 2011 [56]	PK/PD analysis of 2 clinical trials/RA patients	Using a validated electrochemiluminescence assay (refer to the study methods)	- Study 1: placebo or 30-min IV infusion of abatacept, 2 or 10 mg/kg, every 2 week for a total of 7 doses - Study 2: body weight-tiered dose: 30 min IV infusion, 10 mg/kg at 0, 2 and 4 weeks, and then every 1 month	PK parameters of abatacept in Japanese RA pateints
					2 mg/kg (n = 67)			10 mg/kg (n = 61)	Body weight-tiered dose(n = 216)
				Parameter	Mean$\pm$SD
				Cl (L/d)	0.384 $\pm$0.127			0.332 $\pm$0.112	0.299 $\pm$0.079
				AUCss ($\mu$g.h/mL)	7,522 $\pm$2,150			41,005 $\pm$8,410	48,475 $\pm$12,631
				Css-max ($\mu$g/mL)	46.07 $\pm$12.11			225.44 $\pm$38.87	235.85 $\pm$43.18
				Css-min ($\mu$g/mL)	2.87 $\pm$1.55			18.44 $\pm$7.33	24.11 $\pm$10.47
4- Ma, et al., 2009 [62]	Single-center, open-label, clinical PK trial/Healthy volunteers and RA patients	Using enzyme-linked immunosorbent assay (refer to the study materials and methods)	- Healthy volunteers (dose escalation study): 60 min IV infusion of abatacept, 1, 10 or 20 mg/kg single dose - RA patients (single group, multi-dose study): 60 min IV infusion of abatacept, 10 mg/kg, at 0, 2, 4, 8, 12, and 16 weeks	PK parameters of single dose in healthy volunteers
					1 mg/kg (n = 9)		10 mg/kg (n = 9)	20 mg/kg (n = 9)
				Parameter	Mean $\pm$ SD				p-value
				Cl (L/d.kg)	0.0058 $\pm$ 0.0010		0.0067 $\pm$ 0.0010	0.0067 $\pm$ 0.0008	0.119
				Vss (L/kg)	0.126 $\pm$ 0.022		0.138 $\pm$ 0.033	0.114 $\pm$ 0.016	0.143
				t1/2 (d)	15.1 $\pm$ 2.6		14.2 $\pm$ 2.3	11.8 $\pm$ 1.2	0.08
				Cmax (mg/L)	18.4 $\pm$ 1.6		186.9 $\pm$ 24.7	416.8 $\pm$ 34.4	<0.001
				Tmax (d)	0.0416 $\pm$ 0		0.0416 $\pm$ 0	0.0416 $\pm$ 0	>0.05
				PK parameters of multiple doses in RA patients
							10 mg/kg (multiple doses) (n = 8–9)
				Parameter			Mean $\pm$ SD
				Cl (L/d.kg)			0.0050 $\pm$0.0023
				Vss (L/kg)			0.074 $\pm$0.031
				t1/2 (d)			12.6 $\pm$ 4.7
				Cmax (mg/L)			247.1 $\pm$50.3
				Tmax (d)			0.063 $\pm$0.039
				Css-min (mg/L)			239.8 $\pm$45.3
				Css-max (mg/L)			20.5 $\pm$7.9
				AUCss* (mg.d/L)			1830 $\pm$708
				Comparison of PK parameters of CTLA4-Ig in RA patients and healthy volunteers
					10 mg/kg single dose (healthy volunteers) (n = 9)		10 mg/kg multiple doses (RA patients) (n = 8)
				Parameter	Mean $\pm$ SD				p-value
				Cmax (mg/L)	186.9 $\pm$ 24.7		247.1 $\pm$ 50.3		0.006
				t1/2 (d)	14.2 $\pm$ 2.3		12.6 $\pm$ 4.7		0.385
				Cl (L/d.kg)	0.0067 $\pm$ 0.0010		0.0050 $\pm$ 0.0023		0.062
				Vss (L/kg)	0.138 $\pm$ 0.033		0.074 $\pm$ 0.031		0.001
				PD markers affected by multiple dose abatacept in RA patients (10 mg/kg)
				Parameter	Before treatment		16 weeks after treatment		p-value
				TNFα	45.0 $\pm$ 10.5 ng/L		23.8 $\pm$ 3.0 ng/L		<0.05
				MMP-3	58.4 $\pm$ 12.5 μg/L		30.6 $\pm$ 3.4 μg/L		<0.05
				IL-6	30.2 $\pm$ 9.6 ng/L		17.3 $\pm$ 3.3 ng/L		<0.05
5- Srinivas, et al., 1997 [63]	In vivo Pharmacokinetic analysis/Sprague-Dawley rats (n = 48)	Using a validated enzyme immunoassay[57]	- Single dose (IV or SC): 10, 80 or 200 mg/kg - Multiple dose (IV or SC): 10 mg/kg (7 doses over 13 days)		PK parameters after single IV dose of CTLA4-Ig (n = 6)
					Cmax (μg/mL)	$\mathbf{A}\mathbf{U}{\mathbf{C}}_{0-\mathrm{\infty }}$(μg.h/mL)	t1/2 (h)	Cl (mL/h.kg)	Vss (mL/kg)
				Dose	Mean $\pm$ SD
				10 mg/kg	243.4 $\pm$ 27	8857 $\pm$ 1652	64.3 $\pm$ 20.8	1.17 $\pm$ 0.26	149 $\pm$ 15.8
				80 mg/kg	2162 $\pm$ 158	63167 $\pm$ 9012	108 $\pm$ 55.5	1.29 $\pm$ 0.16	193 $\pm$ 21.8
				200 mg/kg	4610 $\pm$ 197	138608 $\pm$ 23256	170 $\pm$ 29.2	1.48 $\pm$ 0.26	243 $\pm$ 32
				Statistical comparison	ND	ND	p < 0.05	p < 0.05	p < 0.05
				PK parameters after single SC dose of CTLA4-Ig (n = 6)
					Cmax (μg/mL)	Tmax (h)	$\mathbf{A}\mathbf{U}{\mathbf{C}}_{0-\mathrm{\infty }}$(μg.h/mL)		t1/2 (h)
				Dose	Mean $\pm$ SD or median (min, max)
				10 mg/kg	26 $\pm$ 6.13	48 (24, 96)	5536 $\pm$ 589		74.4 $\pm$ 16
				80 mg/kg	133.3 $\pm$ 22.6	48 (48, 48)	35153 $\pm$ 8255		132 $\pm$ 26
				200 mg/kg	262.6 $\pm$ 54.5	36 (24, 48)	56900 $\pm$ 11338		167 $\pm$ 26.9
				Statistical comparison	ND	NS	ND		p < 0.05
				PK parameters after multiple IV and SC doses of 10 mg/kg CTLA4-Ig (n = 6)
					Cmax (μg/mL)		AUCtau (μg.h/mL)		t1/2 (h)
				Route	Mean $\pm$ SD
				SC	99.28 $\pm$ 15.7		4340 $\pm$ 680		96.4 $\pm$ 38.4
				IV	414.8 $\pm$ 31.7		9412 $\pm$ 1178		115 $\pm$ 28.5
				Statistical comparison	ND		ND		p < 0.05
6- Srinivas, et al., 1996 [58]	In vivo, parallel design, PK study/skin-grafted and skin-intact mice	Using a validated double antibody sandwich enzyme immunoassay method (refer to the study methods)	- Skin-intact mice group: single IV dose of 0.07, 0.29, or 0.57 mg - Skin-grafted mice group: single or multiple (once a day for 7 days) IV dose of 0.29 mg	PK parameters of the skin-intact and the skin-grafted mice receiving IV bolus doses of CTLA4-Ig†
					Skin-intact mice			Skin-grafted mice
					0.07 mg	0.29 mg	0.57 mg	0.29 mg	0.29 mg (qd$\times$7)
				Parameter	(n = 3)	(n = 3)	(n = 3)	(n = 3)	(n = 3)
					Mean $\pm$ SD
				Cmax ($\mu$g/mL)	56.8 $\pm$ 4.49	290 $\pm$ 24.7	563 $\pm$ 8.36	305 $\pm$ 19.5	523 $\pm$ 42.5
				$\mathrm{A}\mathrm{U}{\mathrm{C}}_{0-\mathrm{\infty }}$ ($\mu$g. h/mL)	1594 $\pm$ 107	8705 $\pm$ 626	15322 $\pm$ 2506	9124 $\pm$ 96.0	8127 $\pm$ 495
				t1/2 (h)	18.8 $\pm$ 11.3	59.2 $\pm$ 2.47	70.4 $\pm$ 7.79	65.7 $\pm$ 21.0	129 $\pm$ 27.9
				ClT (mL/h)	0.04 $\pm$ 0.0	0.03 $\pm$ 0.0	0.04 $\pm$ 0.01	0.03 $\pm$ 0.0	-
				Vss (mL)	2.22 $\pm$ 0.12	2.73 $\pm$ 0.07	3.10 $\pm$ 0.26	2.50 $\pm$ 0.48	-
7- Srinivas, et al., 1996 [59]	In vivo, open, parallel design, multiple dose, PK study/Cynomolgus monkeys	Using a validated enzyme immunoassay (refer to the study materials and methods)	Multiple IV bolus doses of 1, 2.9 or 8.7 mg/kg of CTLA4-Ig on days 1, 4, 8, 11, 15, and 18.	PK parameters after the last IV bolus dose of CTLA4-Ig in monkeys
					1 mg/kg		2.9 mg/kg		8.7 mg/kg
					n = 4		n = 4		n = 4
				Parameter	Mean $\pm$ SD
				Cmax‡ ($\mu$g/mL)	32 $\pm$ 3.4		104 $\pm$ 9.0		270 $\pm$ 20
				AUC0–720§($\mu$g.h/mL)	2783 $\pm$ 528		8812 $\pm$ 1189		25765 $\pm$ 5406
				t1/2 (h)	90 $\pm$ 50		160 $\pm$ 32		135 $\pm$ 29
8- Srinivas, et al., 1998 [64]	PK/PD study/mice	Using a validated enzyme immunoassay[57]	IV treatment: 0.01, 0.04, 0.11, and 0.33 mg SC treatment: 0.06, 0.17, 0.5, 1.6 and 3.3 mg For PK analysis: -Single IV dose of 0.33 mg CTLA4-Ig as a mixture with sheep red blood cells - Single IV dose of 0.33 mg CTLA4-Ig only - Single SC dose of 0.5, 1.6 or 3.3 mg CTLA4-Ig	PK parameters of CTLA4-Ig after single IV or SC dose in mice (n = 6 per group)
					IV		SC
				Parameter	0.33 mg||	0.33 mg¶	0.5 mg	1.6 mg	3.3 mg
				Cmax ($\mu$g/mL)	323	275	96	315	726
				Tmax (h)	0.05	0.05	24	12	9
				$\mathrm{A}\mathrm{U}{\mathrm{C}}_{0-\mathrm{\infty }}$(mg.h/mL)	9.49	8.57	15.8	45.2	73.8
				t1/2 (h)	68	89	87	124	97
				ClT (mL/h)	0.03	0.04	-	-	-
				Vss (mL)	3.4	4.3	-	-	-
				F (%)	-	-	110	98	78
				PD of CTLA4-Ig (suppression of anti-SRBC antibody formation) after single IV or SC dose in mice challenged with SRBC. (n = 5–6/group)
						Inhibition (%)
						Day 12		Day 20	Day 29
				IV dose	0.33	> 99		> 99	> 99
					0.11	98		98	99
					0.04	85		80	81
					0.01	67		65	24
				SC dose	3.3	> 99		> 99	> 99
					1.6	> 99		> 99	> 99
					0.5	> 99		> 99	> 99
					0.17	86		85	69
					0.06	47		27	34

RSE: relative standard error, ND: not determined, NS: not significant, C_max: maximum serum concentration, C_min: minimum serum concentration, T_max: time to the reach C_max, AUC_(tau): area under the concentration–time curve during dosing interval, F: SC bioavailability, Cl: total systemic clearance, Cl_D: distributional clearance, t_1/2: terminal elimination half-life, V₁: volume of central compartment, V₂: volume of peripheral compartment, IC₅₀: drug concentration for 50% maximum inhibition, AUC_ss: area under the concentration–time curve at steady-state, V_ss: volume of distribution at steady state, C_ss-max: steady-state peak concentration, C_ss-min: steady-state trough concentration, $\mathrm{A}\mathrm{U}{\mathrm{C}}_{0-\mathrm{\infty }}:$area under the serum concentration–time curve from time 0 to infinity, AUC_tau: area under the serum concentration–time curve for a dosing interval, TNFα: Tumor necrosis factor alpha, MMP-3: matrix metalloproteinase-3, IL-6: interleukin-6, SRBC: sheep red blood cells.

*: AUC_ss here is the observed AUC_{0–28 d} after the last dose at steady state.

†: No statistical significance in the PK parameters between the both groups after a single 0.29-mg dose of CTLA4-Ig.

‡: The concentration observed from the first sampling point (10 min).

§: Area under the concentration–time curve immediately after the last dose (day 18) to the last measurable concentration.

||: Treatment was administered after sheep red blood cell (SRBC) injection.

¶: mixture of CTLA4-Ig and sheep red blood cells administered together.

Table 3. Mean values for selected pharmacokinetic parameters of CTLA4-Ig (abatacept).

No.	Ref.	Administered Product & Dosage Regimen*	Species	n	AUC0-inf (g·h/L)	AUC (g·h/L)	Cl (L/h)	Cl (L/h/kg)	Cmax (g/L)	Cmin  (g/L)	F (%)	ka (h^-1)	MRT (h)	Q (L/h)	Q (L/h/kg)	t1/2  (h)	Tmax (h)	Vd(c)  (L)	Vd(c) (L/kg)	Vd(p) (L)	Vd(p) (L/kg)	Vss (L)	Vss (L/kg)
1	[58]	Abatacept 0.29 · 10‾^3 g IV o.d. x 7 d	Mice; Female	3	8.127				0.523							129
2	[58]	Abatacept 70 · 10‾⁶ g IV s.d.	Mice; Female	3	1.594		0.00004		0.0568				49.4			18.8						0.0022
3	[58]	Abatacept 0.29 · 10‾^3 g IV s.d.	Mice; Female	3	8.705		0.00003		0.29				83			59.2						0.0027
4	[60]	Abatacept 0.29 · 10‾^3 g IV s.d.	Mice; Female	8	8.384		0.000035		0.281				113			90						0.0039
5	[64]	Abatacept 0.33 · 10‾^3 g IV s.d.	Mice; Female	6	9.49		0.00003		0.323				97			68						0.0034
6	[58]	Abatacept 0.57 · 10‾^3 g IV s.d.	Mice; Female	3	15.322		0.00004		0.563				82.2			70.4						0.0031
7	[60]	Abatacept 0.29 · 10‾^3 g SC s.d.	Mice; Female	8	7.2				0.0592		85		116			77	12
8	[64]	Abatacept 0.5 · 10‾^3 g SC s.d.	Mice; Female	1	15.8				0.096		110		135			87	24
9	[64]	Abatacept 1.6 · 10‾^3 g SC s.d.	Mice; Female	1	45.2				0.315		98		173			124	12
10	[64]	Abatacept 3.3 · 10‾^3 g SC s.d.	Mice; Female	1	73.8				0.726		78		151			97	9
11	[63]	Abatacept 10 · 10‾^3 g/kg IV 1x/2 d x 13 d	Rats	6		9.412			0.415							115
12	[55]	Abatacept 10 · 10‾^3 g/kg IV s.d.	Rats	5				0.0009							0.0011				0.0695		0.0619
13	[63]	Abatacept 10 · 10‾^3 g/kg IV s.d.	Rats	6	8.857			0.0117	0.243				131			64.3							0.149
14	[63]	Abatacept 80 · 10‾^3 g/kg IV s.d.	Rats	6	63.167			0.0129	2.162				152			108							0.193
15	[63]	Abatacept 0.2 g/kg IV s.d.	Rats	6	138.608			0.0148	4.61				167			170							0.243
16	[63]	Abatacept 10 · 10‾^3 g/kg SC 1x/2 d x 13 d	Rats	6		4.34			0.0993							96.4
17	[55]	Abatacept 20 · 10‾^3 g/kg SC s.d.	Rats	6				0.0009			59.2				0.0011				0.0695		0.0619
18	[63]	Abatacept 10 · 10‾^3 g/kg SC s.d.	Rats	6	5.536				0.026				165			74.4	48
19	[63]	Abatacept 80 · 10‾^3 g/kg SC s.d.	Rats	6	35.153				0.1				232			132	48
20	[63]	Abatacept 0.2 g/kg SC s.d.	Rats	6	56.9				0.3				223			167	36
21	[59]	Abatacept 1 · 10‾^3 g/kg IV 2x/wk x 3 wk	Monkeys	4		2.783			0.032							90
22	[59]	Abatacept 2.9 · 10‾^3 g/kg IV 2x/wk x 3 wk	Monkeys	4		8.812			0.104							160
23	[59]	Abatacept 8.7 · 10‾^3 g/kg IV 2x/wk x 3 wk	Monkeys	4		25.765			0.27							135
24	[54]	Abatacept 10 · 10‾^3 g/kg IV 1x/2 wks	Humans; Adult	574			0.0217							0.0212				3.1		4.1
25	[61]	Abatacept 0.5 · 10‾^3–10 · 10‾^3 g/kg IV 1x/4 wks	Humans	2244			0.0204							0.0265		116		3.3		4.3
26	[54]	Abatacept 0.5 · 10‾^3 g/kg IV s.d.	Humans; Adult	574			0.0217							0.0212				3.1		4.1
27	[54]	Abatacept 2 · 10‾^3 g/kg IV s.d.	Humans; Adult	574			0.0217							0.0212				3.1		4.1
28	[54]	Abatacept 10 · 10‾^3 g/kg IV s.d.	Humans; Adult	574			0.0217							0.0212				3.1		4.1
29	[54]	Abatacept 0.5 g IV s.d.	Humans; Adult	574			0.0217							0.0212				3.1		4.1
30	[54]	Abatacept 0.75 g IV s.d.	Humans; Adult	574			0.0217							0.0212				3.1		4.1
31	[54]	Abatacept 1 g IV s.d.	Humans; Adult	574			0.0217							0.0212				3.1		4.1
32	[56]	Abatacept 2 · 10‾^3 g/kg IV s.d. x 30 min	Humans; Asian; Adult	344		7.5	0.0144		0.046	0.003				0.006				2.7		2.1
33	[56]	Abatacept 10 · 10‾^3 g/kg IV s.d. x 30 min	Humans; Asian; Adult	344		41	0.0144		0.225	0.018				0.0 06				2.7		2.1
34	[61]	Abatacept 0.075–0.2 g SC 1x/wk	Humans	2244			0.0204				80.5	0.0031		0.0265					3.3		4.3

S.D. = single dose, o.d. = once daily, $\mathbf{A}\mathbf{U}{\mathbf{C}}_{0-\mathbf{i}\mathbf{n}\mathbf{f}}:$area under the serum concentration–time curve from time 0 to infinity, AUC: area under the concentration–time curve, Cl: total systemic clearance, C_max: maximum serum concentration, C_min: minimum serum concentration, F: SC bioavailability, ka = absorption rate constant, MRT = mean residence time, Q = distributional clearance, t_1/2: terminal elimination half-life, T_max: time to the reach C_max, Vd_(c): volume of central compartment, Vd_(p): volume of peripheral compartment, V_ss: volume of distribution at steady state.

Abatacept administered intravenously was evaluated in healthy volunteers receiving a single dose of 10 mg/kg compared to RA patients receiving multiple doses of 10 mg/kg, and showing linearity over a dose range 1–20 mg/kg. Peak plasma concentration of abatacept (C_max) was 186.9 $\pm$24.7 and 247.1 $\pm$50.3 mg/L (p = 0.006) and volume of distribution (V_ss) was 0.138 $\pm$0.033 and 0.074 $\pm$0.031 L/kg (p = 0.001) in healthy volunteers and RA patients, respectively [62]. In vivo, treatment of Sprague-Dawley rats with multiple 10 mg/kg doses of SC abatacept resulted in significantly lower terminal half-life (t_1/2) compared to IV multiple doses [63].

Pharmacodynamic pro-inflammatory markers that are elevated in RA patients such as TNFα, MMP-3, and IL-6 were significantly reduced at 16-weeks after treatment in RA patients [62]. In addition, mice challenged with sheep red blood cells (SRBC) in addition to the IV or SC single dose of abatacept had a significant percentage of inhibition of anti- SRBC antibodies [64].

5. Conclusion

Abatacept was tested for the prevention of complete loss of beta-cell function in patients recently diagnosed with type 1 diabetes. It showed good efficacy in conserving beta-cell function through the preservation of C–peptide levels and improvement of insulin sensitivity, but only for a transient period of time. Abatacept has a good efficacy profile, as shown in large post-marketing surveillance studies. New routes of administrations should be made available to alleviate the discomfort associated with IV and SC routes of abatacept. In addition, future studies should investigate the use of combinational therapy to preserve the treatment effect for a longer duration.

6. Expert opinion

Immunomodulation is one of the promising strategies in slowing down the progression of beta-cell destruction after the onset of type 1 diabetes. It also holds the advantage of specific immune modulation without systemic general immunosuppression. In a series of The Lancet journals, immunological strategies were shown to fundamentally change the management of type 1 diabetes [65]. Systemic suppression of the immune system results in a long-term serious complication, which is the major concern that limits the use of immunosuppressants such as cyclosporine in recently diagnosed type 1 diabetes [66,67]. Therefore, newer therapeutic agents have emerged to overcome this problem. For example, etanercept, a TNF-α antagonist, has been shown to be effective in increasing beta-cell function compared to the control; however, it is associated with serious infections [68]. In addition, teplizumab and otelixizumab are anti-CD3, which target the destructive T-cells to delete them and regenerate new nondestructive T-cells that have been tested in type 1 diabetes patients. Although teplizumab was unsuccessful in meeting primary end points of insulin use of less than 0.5 unit/kg/day and HbA1c of less than 6.5% at 1 year [69], it preserved the secretion of C-peptide by 30% and showed significant efficacy after 2 years of follow up in young patients with good residual insulin production. Also, there was no substantial increase in adverse events compared to placebo [70]. However, in a recent phase 2 clinical trial of teplizumab treatment in relatives with type 1 diabetes, clinically important and statistically significant adverse events associated with teplizumab were reported, and the beneficial use of teplizumab in delaying progression to clinical type 1 diabetes in high-risk participants is still under debate [71,72].

The other agent, otelixizumab, has also shown efficacy, but it appears to have a narrow therapeutic window. It did not result in the conservation of beta cells when used at a low dose [73]. On the other hand, when the dose was increased, it showed significant preservation of beta-cell function at 18 months; however, it was associated with serious adverse events [74]. Another tested agent is rituximab, an anti-CD20 that downregulates B lymphocytes. It showed some efficacy but for a short period [75,76].

Abatacept, the agent reviewed in this study, is an immunomodulatory agent that blocks the costimulation of T-cells, which help in treating the autoimmune destruction of beta cells in type 1 diabetes. It has been shown to slow the rate of the reduction of beta-cell function in patients with recent onset of type 1 diabetes. In addition, the reduction of central memory CD4⁺ T cells by abatacept can be used as a replacement marker for C-peptide reduction in recently diagnosed type 1 diabetes patients [77]. Hence, it can be useful for monitoring the efficacy of treatment interventions. This is particularly important, as the change in fasting C-peptide level was limited in the first 48 months compared to baseline, making this outcome of limited reliability in clinical trials to demonstrate benefit of a potential therapy [78]. However, the immune tolerability of abatacept is not long-lasting and may be associated with resistance due to B lymphocyte alterations. Moreover, the short-period effect appears to coincide with the fact that autoimmunity subsides with time in type 1 diabetes patients .

Another strategy, which might be effective in prolonging the effect of the treatment, could be the use of combination therapy. These primary prevention strategies must be safe and well-tolerated because they are administered to healthy individuals who have no clinical signs of autoimmune disease or metabolic impairment, and whose risk of developing type 1 diabetes is unknown. Therefore, an ongoing trial (Type 1 Diabetes TialNet, which is currently Closed to Enrollment – Study Ongoing) is examining the effect of abatacept in preventing progression to clinical type 1 diabetes in Stage 1 & 2 patients with islet autoantibodies [79]. Variations in response to CTLA4-Ig might be partially explained by the new insights of the existence of multiple type 1 diabetes subtypes with varying baseline innate inflammatory/regulatory bias and the rate of C-peptide decline [80]. Heterogeneity of C-peptide decline was observed between individuals after the clinical diagnosis of type 1 diabetes, which was evidenced by the variability in C-peptide slope over time. Factors that may have contributed to such variability included age and number of basophils [78]. Further investigation of the progression of type 1 diabetes in different age groups warrants better understanding and enhancement of population response to immunomodulation therapy.

In a study exploring the possible mechanism of abatacept resistance, abatacept-treated subjects were grouped into respondents (Rs) and non-respondents (NRs) based on the rate of the progression of type 1 diabetes, measured by C-peptide decline [81]. It was concluded that differential expression of B-cell genes, either individually or modularly, can be used to predict disease progression in both R and NR individuals of the abatacept-treated group after 84 days. These differences seemed to be transient and subsided after 2 years and were not evident when the abatacept-treated group was compared to placebo. Furthermore, the transient B-cell gene expression differences were linked to differential expression of T cell costimulatory ligands including ICOSLG, CD40, and CD58 between R and NR groups. This suggests a reprogramming mechanism of the CD28/CD80/CD86 T cell co-stimulation in response to abatacept therapy, leading to the observed resistance in the poor response group. This study together with others suggest the association of B cells, T cells, neutrophils, and other cell types with abatacept response and resistance in R and NR groups, respectively [77,80,81].

A number of other factors contributing to unsuccessful immunomodulation in type 1 diabetes trials have been identified including disease heterogeneity, inappropriate preclinical models, poor understanding of the disease immunological and metabolic conditions, flaws in trial designs, and the need for further understanding of type 1 diabetes pathogenesis [77,78,81]. Despite the above-mentioned factors, our collective and accumulative understanding of the pathogenesis of type 1 diabetes and related physiological status, augmented by the new advances in the designs and models used in preclinical and clinical trials to understand the underlying immunological and metabolic status, get us one step closer to an immunomodulation therapy and increase our hopes for better management of type 1 diabetes.

7. Drug Summary

Drug Summary

Drug name	CTLA4-Ig (abatacept)
Current indication	Adult rheumatoid arthritis, juvenile idiopathic arthritis, and adult psoriatic arthritis
Orphan drug designation	By U.S. Orban Biotech LLC (Massachusetts, USA) for the treatment of type 1 diabetes mellitus patients with residual beta cell function.
Mechanism of action	It is a selective costimulation modulator, inhibiting T cell (T lymphocyte) activation by binding to CD80 and CD86, thereby blocking interaction with CD28. This interaction provides a costimulatory signal necessary for full activation of T lymphocytes
Route of administration	Intravenous or subcutaneous
Clinical trials including the use of abatacept for type 1 diabetes (ClinicalTrials.gov): 1. NCT03929601: Rituximab and abatacept for prevention or reversal of type 1 diabetes (TN25) – not yet recruiting. 2. NCT01773707: CTLA4-Ig (abatacept) for prevention of abnormal glucose tolerance and diabetes in relatives at risk for type 1 diabetes – recruiting. 3. NCT00505375: Intravenous CTLA4-lg treatment in recent onset type 1 diabetes mellitus – completed. 4. NCT04118153: Early Markers of Disease and Response to Therapy – not yet recruiting. 5. NCT00276250: Islet transplantation using abatacept – completed. 6. NCT00501709: Prevention of autoimmune destruction and rejection of human pancreatic islets following transplantation for insulin dependent diabetes mellitus – completed.

Article highlights

• CTLA4-Ig slows the rate of the reduction of beta-cell function in patients with recent onset of type 1 diabetes.

• Available evidence suggests that T-cell co-stimulation blocking by CTLA4-Ig is effective in prolonging the survival of islet transplants in vivo and associated with much lower side effects than general immunosuppression.

• CTLA4-Ig was able to slow the reduction of beta-cell function early in the development of the disease (within 3 months of diabetes), and the effect was continued 1 year after the cessation of treatment but without much improvement.

• The addition of other agents can be considered for a synergistic effect in prolonging allograft survival, particularly islet allografts.

• The reduction of central memory CD4⁺ T cells by CTLA4-Ig can be used as a replacement marker for C-peptide reduction in recently diagnosed type 1 diabetes patients.

• The immune tolerability of CTLA4-Ig is not long-lasting and may be associated with resistance due to B lymphocyte alterations. The effect appears to be for a short period of time since the autoimmunity subsides with time in type 1 diabetes patients.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

One reviewer has served on advisory boards for Caladrius, Immunomolecular Therapeutics and Tolerion and had an affiliation with Novo Nordisk. Peer reviewers on this manuscript have no other relevant financial or other relationships to disclose.

Additional information

Funding

This work was made possible by the National Priorities Research Program award [NPRP9-350-3-074] from the Qatar National Research Fund (a member of The Qatar Foundation). The contents herein are solely the responsibility of the author.