Anticancer and cardio-protective effects of liposomal doxorubicin in the treatment of breast cancer

Abstract

Breast cancer (BC) is a highly prevalent disease, accounting for the second highest number of cancer-related mortalities worldwide. The anthracycline doxorubicin (DOX), isolated from Streptomyces peucetius var. caesius, is a potent chemotherapeutic drug that is successfully used to treat various forms of liquid and solid tumors and is currently approved to treat BC. DOX exerts its effects by intercalation into DNA and inhibition of topoisomerases I and II, causing damage to DNA and the formation of reactive oxygen species (ROS), resulting in the activation of caspases, which ultimately leads to apoptosis. Unfortunately, DOX also can cause cardiotoxicity, with patients only allowed a cumulative lifetime dose of 550 mg/m². Efforts to decrease cardiotoxicity and to increase the blood circulation time of DOX led to the US Food and Drug Administration (FDA) approval of a PEGylated liposomal formulation (L-DOX), Doxil^® (known internationally as Caelyx^®). Both exhibit better cardiovascular safety profiles; however, they are not currently FDA approved for the treatment of metastatic BC. Here, we provide detailed insights into the mechanism of action of L-DOX and its most common side effects and highlight results of its use in clinical trials for the treatment of BC as single agent and in combination with other commonly used chemotherapeutics.

Keywords:

• doxil
• caelyx
• breast cancer
• anti-tumor activity
• cardiotoxicity

Introduction

Breast cancer (BC) is the second most frequent cause of cancer-related deaths in women worldwide. It is a heterogeneous disease composed of multiple subtypes with distinct pathological features and clinical implications. Although men are affected, to a lesser degree, the most significant risk factors are gender (women) and older age. Other risk factors include obesity, estrogen exposure, alcohol consumption, and a family history.¹ Over the past 2 decades, accumulating evidence, both clinical and experimental, has suggested that BCs with different histopathological and biological features exhibit distinct behaviors that lead to different treatment responses and, therefore, should be given different therapeutic strategies.² On this basis, at diagnosis, BC patients are systematically tested for the presence of receptors, including estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), in order to explore tailored treatment options with molecularly targeted therapies. However, for patients who are triple negative (ER-, PR-, and HER2-), those who have innate or acquired resistance to targeted therapies, and patients whose disease has metastasized, traditional treatment options including surgery, radiotherapy, and chemotherapy are favored.

Anthracycline-based chemotherapy with doxorubicin (DOX) is one of the most efficacious anticancer agents for both early- and late-stage BCs.³ DOX’s mechanism of action (Figure 1) on cancer cells begins with its passive diffusion through the phospholipid bilayer membrane of malignant cells into the cytoplasm, where DOX is converted into a semiquinone and generates reactive oxygen species (ROS), causing free radical formation and oxidative stress. In the cytosol, DOX enters the mitochondria causing DNA damage and energetic stress. As a result, the mitochondria release the cytochrome C protein, triggering the caspase cascade leading to cell death. From the cytosol, DOX translocates into the nucleus where it intercalates between double-stranded DNA helices and inhibits the enzymes topoisomerases I and II. The resulting damage to DNA leads to free radical generation, alkylation, and activation of the p53 pathway, hence inhibiting cell proliferation and inducing apoptosis. DOX can also hyperactivate the nuclear enzyme poly ADP ribose polymerase (PARP)-1, hence depleting the cell’s energy, thereby resulting in autophagy.^4–6

Figure 1 Mechanism of action of DOX and L-DOX.

Notes: Once L-DOX diffuses across the phospholipid bilayer of malignant cells, free DOX is released and it can be converted into a semiquinone or translocate into the nucleus or mitochondria. Conversion of DOX to a semiquinone causes the formation of ROS. DOX can also translocate to the nucleus where it intercalates between strands of DNA, inhibits topoisomerases I and II and activates PARP-1. In the mitochondria, DOX intercalates between strands of mitochondrial DNA and leads to the release of cytochrome C and the activation of caspases. Ultimately, damage to nuclear and mitochondrial DNA as well as that caused by ROS leads to apoptosis.
Abbreviations: DOX, doxorubicin; L-DOX, liposomal DOX; PARP, poly ADP ribose polymerase; ROS, reactive oxygen species.

However, the potential therapeutic benefits of DOX are limited by the risk of cardiotoxicity, which has been evidently related to its lifetime cumulative dose.^7–9 To overcome this hurdle, the liposomal DOX (L-DOX) formulation was developed in order to reduce DOX-associated cardiotoxicity while preserving its antitumor efficacy.¹⁰ The L-DOX formulation encapsulates DOX within a phospholipid bilayer that is coated with methoxypolyethylene glycol (Figure 2). The PEGylation protects the liposomes from recognition by the mononuclear phagocyte system (MPS) and allows a longer circulation time in the bloodstream while reducing the exposure of free DOX circulating in the plasma. Biodistribution studies have shown that L-DOX has the ability to deposit and/or penetrate tumors and release DOX.¹¹ Although the mechanism of release of DOX from its liposomes is still unknown, a 10-fold higher exposure of L-DOX than DOX is observed in metastatic BC tissue as compared to healthy breast tissue,¹² which is explained by the enhanced permeability retention effect.¹³ This indicates that a therapeutic approach with L-DOX is more targeted compared to DOX. A differential pharmacokinetic (PK) characteristic between the two formulations includes a decreased clearance (CL), a smaller volume of distribution, and a longer half-life for L-DOX compared to DOX as a result of the sequestration of liposomes, due to their increased size, in the sinusoidal lumen of the liver, which limits their flow from the fenestrations of the lumen into the hepatocytes as compared to free DOX. This is also believed to contribute to reduced hepatic extraction of L-DOX compared to DOX as a single agent.¹⁴ The latter being primarily metabolized in the liver to the major cytotoxic metabolites: doxorubicinol and cytotoxic aglycones. The PK properties of L-DOX as compared to DOX are summarized in Table 1.

Figure 2 PEGylated liposomal DOX (Doxil^®).

Notes: DOX is surrounded by a phospholipid bilayer (liposome) that is coated with methoxypolyethylene glycol. Enclosing DOX in a liposome helps decrease systemic side effects while PEGylation protects the liposomes from recognition by the mononuclear phagocyte system and increases its circulation time.
Abbreviation: DOX, doxorubicin.

Table 1 Pharmacokinetic properties of DOX and L-DOX

Parameter (unit)	Definition	DOX, min–max^Citation35	L-DOX, average ± SD^Citation36
CL (mL/min/m^2)	Plasma clearance	324–809	0.683±0.066
T1/2 (h)	Half-life	20–48	55±4.8
Vd (L/m^2)	Volume of distribution	809–1,214	2.72±0.12
Fb (%)	Fraction bound to plasma proteins	75a	Not determined

Note:

a Data shown as average.

Abbreviations: CL, clearance; DOX, doxorubicin; L-DOX, liposomal DOX; SD, standard deviation.

The use of DOX as a stand-alone treatment has shown effectiveness in overall survival and response rate and time to disease progression.¹⁵ Its limitations are seen in its effect on healthy cells and the resulting adverse effects, with the most significant being cardiotoxicity. The comparison of safety profiles between DOX and L-DOX is summarized in Table 2 with the most common side effect for L-DOX being palmar–plantar erythrodysesthesia, a skin toxicity that can be managed with supportive care, unlike cardiotoxicity, which is significantly more prevalent for DOX.¹⁶ Although the mechanism of DOX-induced cardiotoxicity is not fully understood, its administration is dose limited. It was shown that the risk of DOX-induced cardiotoxicity increases with the increase in its cumulative lifetime dose to becoming irreversible (ie, cardiomyocytes death) when the latter reaches 450–550 mg/m².¹⁷ The other major cardiotoxic effects of DOX are the occurrence of congestive heart failure (CHF) in >20% of the treated patients.⁸ Additional but less detrimental adverse reactions include nausea, vomiting, gastrointestinal problems, neurological symptoms, and cutaneous injuries at the site of injection.⁴

Table 2 Comparison of use, efficacy, and safety profiles of DOX and L-DOX

Criteria	DOX^Citation16,Citation21,Citation23	L-DOX^Citation16,Citation22,Citation24
Indications	Label: leukemia, bladder cancer, breast cancer, gastric cancer, sarcomas, small cell lung cancer among others	Label: Kaposi’s sarcoma, multiple myeloma, and ovarian cancer
	Off label: biliary tract cancer, endometrial cancer,	Off label: use for breast cancer, cutaneous
	Kaposi’s sarcoma, malignant melanoma	T-cell lymphoma
Maximum dose limits	Lifetime: up to 550 mg/m^2 IV	50 mg IV weekly
	Up to 450 mg IV if previous mediastinal radiation	Safety and efficacy have not been established in children and adolescents
Black box warnings	Bone marrow suppression, cardiotoxicity, extravasation	Cardiotoxicity and infusion-related reactions
Adverse effects	Cardiotoxicity	Palmar–plantar erythrodysesthesia
	Nausea/vomiting	Nausea/vomiting
	Alopecia	Alopecia
	Leukopenia/neutropenia	Cardiotoxicity
	Palmar–plantar erythrodysesthesia	Leukopenia/neutropenia

Abbreviations: DOX, doxorubicin; L-DOX, liposomal DOX; IV, intravenous.

While L-DOX has proven to be advantageous over DOX as a single agent, it is only indicated in the treatment of metastatic BC in combination with docetaxel,¹⁸ although it is frequently utilized as adjuvant therapy in metastatic BC.¹⁹ The comparative anticancer efficacies of these two drugs in addition to their use in combination with other anticancer agents in BC continue to be investigated.²⁰Table 3 summarizes the main findings of trials assessing regimens containing L-DOX. These trials evaluated the efficacy of several combinations in varied patient populations including both locally advanced and metastatic BCs as well as the elderly and in patients with previously treated BCs. Overall, L-DOX was effective and well tolerated in the majority of trials, making it a feasible treatment option when combined with other chemotherapeutics. In the present work, we sought to investigate L-DOX’s efficacy on BC and its potential associated cardiac toxicity when combined with other chemotherapeutic or targeted therapy, with the goal of showcasing the beneficial effect of L-DOX over DOX on BC therapy and cardiac function.

Table 3 Clinical trials with L-DOX in combination with other chemotherapeutics in breast cancer

Drugs	Clinical trial	Main findings	References
PLD and bevacizumab	Phase II	46% of patients had grade three toxicities	^Citation37
- 20 mg/m^2 PLD and 10 mg/kg bevacizumab given days 1 and 15 of 4-week cycle		ORR: 21%
		Median PFS: 5.7 months
		Median OS: 15.9 months
		Severe cardiotoxicity in one patient 4.7 months after treatment
PLD and bortezomib	Phase II	ORR: 8%	^Citation38
- 1.3 mg/m^2 bortezomib days 1, 4, 8, and 11 and 30 mg/m^2 PLD day 4 of 21-day cycle		Median OS: 4.3 months
		Median TTP: 1.3 months
		No cardiotoxicity reported despite prior anthracycline use in 77% of patients
PLD and carboplatin (+ trastuzumab if HER2+)	Phase II	PLD/carboplatin	^Citation39
		1A: taxane naive
30 mg/m^2 PLD and carboplatin AUC = 5 mg^*min/mL day 1 of 21-day cycle		ORR: 31%
		Median PFS: 8 months
If HER2+ also got 8 mg/kg loading dose of trastuzumab and then 4 mg/kg days 1 and 15 of 28-day cycle		1B: taxane pretreated
		ORR: 31%
		Median OS: 13 months
		Median PFS: 5 months
		PLD/carboplatin/trastuzumab
		ORR: 56%
		Median OS: 33 months
		Median PFS: 10 months
		No clinically significant cardiotoxicity. Declines in LVEF of at least
		15% in two patients in arm 1A and one patient in trastuzumab arm
PLD, cisplatin, infusional 5-FU, and trastuzumab	Phase II	Clinical response rate: 94%	^Citation31
		2-year DFS: 94%
25 mg/m^2 PLD, 60 mg/m^2 cisplatin day 1 and 200 mg/m^2 5-FU as a continuous infusion days 1–21 of 21-day cycle 8 mg/kg trastuzumab loading dose then 6 mg/kg on day 2 of cycle		No relevant cardiotoxicity – two patients had asymptomatic, transient declines in LVEF of at least 20%, but absolute LVEF was maintained above 50% in both cases
PLD and cyclophosphamide	Phase II	Objective response: 51%	^Citation40
Cohort I: 50 mg/m^2 PLD day 1 and 100 mg/m^2 cyclophosphamide orally days 1–14 (28-day cycle)		Clinical benefit rate (complete response + partial response + stable disease): 86%
		Median DOR: 35.1 weeks
Cohort II: 30 mg/m^2 PLD and 600 mg/m^2 cyclophosphamide day 1 of 21-day cycle		Median time-to-tumor progression: 34.4 weeks
		No clinical cardiotoxicity or significant declines in median LVEF following treatment
Cohort III: 35 mg/m^2 and 600 mg cyclophosphamide day 1 of 21-day cycle
PLD and cyclophosphamide (in elderly patients between 65–75 years old)	Phase II	Objective response rate: 28.6%	^Citation41
		Median PFS: 8.8 months
- 40 mg/m^2 PLD and 500 mg/m^2 cyclophosphamide day 1 of 4-week cycle		Median OS: 20.3 months
		Mucositis, myelosuppression in the elderly
		No reported cardiac toxicity or significant changes in LVEF
PLD, cyclophosphamide, and 5-FU	Phase II	ORR: 41.9%	^Citation42
- 40 mg/m^2 PLD, 500 mg/m^2 cyclophosphamide, and 500 mg/m^2 5-FU on day 1 of 21-day cycle		Median PFS: 8.2 months
		Median OS: 36.6 months
		No significant changes in LVEF, even after prior anthracycline exposure
PLD, cyclophosphamide, and paclitaxel	Phase II	Overall pCR: 32%	^Citation43
- 35 mg/m^2 PLD, 600 mg/m^2 cyclophosphamide every 4 weeks, and 80 mg/m^2 paclitaxel weekly		Radiological ORR: 26%
		5-year PFS: 58%
		5-year OS: 62%
		No significant declines in LVEF or ECG changes; five cardiac events reported
PLD, cyclophosphamide, and trastuzumab	Phase II	Objective response rate: 68.8%	^Citation27
		Median OS: 34.2 months
50 mg/m^2 PLD and 600 mg cyclophosphamide every 4 weeks		Median TTP: 12 months
		No symptomatic CHF; declines in LVEF observed in eight of the
4 mg/kg trastuzumab loading dose then 2 mg/kg weekly		48 patients and reversed in seven patients (six of them had prior anthracycline exposure)
PLD, cyclophosphamide, docetaxel, and trastuzumab (if HER2+)	Phase II	Objective response rate: 83%	^Citation44
		pCRT: 13%
35 mg/m^2 PLD and 600 mg cyclophosphamide on day 1 of 21-day cycle		Normal LVEF maintained during the study
100 mg/m^2 docetaxel day 1 of 21-day cycle
If HER2+ got 8 mg/kg loading dose of trastuzumab then 6 mg/kg day 1 of 21-day cycles
PLD and docetaxel	Phase II	Overall clinical benefit: 47%	^Citation45
Group A: 35 mg/m^2 PLD on day 1 and 40 mg/m^2 docetaxel days 1 and 15 of 28-day cycle		ORR: 49%
		High rates of toxicity in both groups
		No significant cardiotoxicity
Group B: 30 mg/m^2 PLD day 1 and 75 mg/m^2 docetaxel day 2 (3-week cycle)
PLD and docetaxel (trastuzumab if HER2+)	Phase II	PLD/docetaxel	^Citation46
		ORR: 47.4%
Group A: 30 mg/m^2 PLD and 60 mg/m^2 docetaxel every 3 weeks		Median PFS: 11 months
		Median OS: 24.6 months
Group B: 30 mg/m^2 PLD, 60 mg/m^2 docetaxel every 3 weeks, and 4 mg/kg trastuzumab loading dose then 2 mg/ kg weekly		PLD/docetaxel/trastuzumab
		ORR: 45.7%
		Median PFS: 10.6 months
		Median OS: 31.8 months
		Higher rates of hand foot syndrome
		Incidence of CHF <3% and the addition of trastuzumab did not increase CHF risk
PLD and docetaxel vs docetaxel (in patients who experienced a relapse following adjuvant anthracycline use)	Phase III	PLD/docetaxel	^Citation47
		Objective response rate: 35%
		Median TTP: 9.8 months
- 30 mg/m^2 PLD and 60 mg/m^2 docetaxel (21-day cycle) vs 75 mg/m^2 docetaxel (21-day cycle)		Median OS: 20.5 months
		Docetaxel
		Objective response rate: 26%
		Median TTP: 7 months
		Median OS: 20.6 months
		No significant increase in CHF incidence or LVEF decline with the addition of PLD
PLD and GEM	Phase II	ORR: 47.8%	^Citation48
- 25 mg/m^2 PLD day 1 and 800 mg/m^2		Median TTP: 7 months
GEM on days 1 and 8 of 21-day cycle		Median duration of clinical benefit: 8 months
		Mild cardiac toxicity in 4% patients; it was recovered after the end of the study
PLD and GEM	Phase II	ORR: 52%	^Citation49
- 24 mg/m^2 PLD day 1 and 800 mg/m^2 GEM days 1 and 8 (21-day cycle)		Median OS: 16.1 months
		Median TTP: 4.5 months
		Clinical benefit: 78%
		Minimal cardiotoxicity, with a transient decline in LVEF in one patient, who recovered after the end of the study
PLD and GEM	Phase II	ORR: 39.1%	^Citation50
- 25 mg/m^2 PLD and 800 mg/m^2 GEM days 1 and 8 of 21-day cycle		Median TTP: 11 months
		Overall clinical benefit: 85.9%
		Only one case of cardiac toxicity was observed, despite 41% of patients having undergone prior anthracycline therapy
PLD, GEM and docetaxel	Phase II	ORR: 74%	^Citation51
- 1,000 mg/m^2 GEM on day 1 followed by 800 mg/m^2 GEM, 75 mg/m^2 docetaxel, and 30 mg/m^2 PLD on day 8 (3-week cycles)		No cardiotoxicity was observed as per protocol-defined criteria and all patients maintained LVEF >50%
PLD and lapatinib	Phase II	ORR: 54%	^Citation52
- 1,250 mg lapatinib daily and 40 mg/m^2		Median PFS: 5.8 months
PLD every 4 weeks		Median OS: 23.3 months
		No cardiac events were observed
PLD (low dose) and paclitaxel	Phase II	Objective response rate: 74%	^Citation53
- 15 mg/m^2 PLD every other week and 80 mg/m^2 paclitaxel weekly		9% pathological complete response on breast and axilla
		55% had breast conserving surgery
		No impairment of cardiac function was observed
PLD and paclitaxel	Phase II	Objective response rate: 80%	^Citation54
- 30 mg/m^2 PLD and 175 mg/m^2 paclitaxel (3-week cycles)		Median duration of objective response: 31 weeks
		Median time to treatment failure: 45 weeks
		Decline in LVEF was observed in eight of the 26 patients; however, no clinical signs or symptoms of cardiac toxicity/failure were observed
PLD and trastuzumab	Phase II	Clinical benefit: 50%	^Citation30
- 40 mg/m^2 PLD every 4 weeks and 4 mg/kg trastuzumab loading dose then 2 mg/kg weekly		Median PFS: 9.67 months
		Median OS: 16.23 months
		Three of the 16 patients developed decline in LVEF; a clinically relevant and symptomatic decrease occurred in only one patient
PLD and trastuzumab	Phase II	ORR: 52%	^Citation28
- 50 mg/m^2 PLD every 4 weeks and 4 mg/kg trastuzumab loading dose then 2 mg/kg weekly		Median DOR: 11.1 months
		Median PFS: 12 months
		10% of the patients developed protocol-defined cardiotoxicity, albeit without any symptoms of CHF
PLD and trastuzumab	Phase II	ORR: 22%	^Citation55
- 30 mg/m^2 PLD and 8 mg/kg trastuzumab loading dose then 6 mg/kg (3-week cycles)		Median PFS: 6.5 months
		Median OS: 18.7 months
		Severe cardiotoxicity as per protocol-defined criteria was not recorded. Median LVEF was maintained at 62% throughout the study
PLD and VNB	Phase II	Objective response rate: 36%	^Citation56
- 30 mg/m^2 PLD day 1 and 20 mg/m^2		Median PFS: 6.7 months
VNB on days 1 and 8 (3-week cycles)		Median OS: 13.2 months
		No cardiotoxicity was observed
PLD and VNB	Phase II	ORR: 39%	^Citation29
- 40 mg/m^2 PLD day 2 and 25 mg/m^2		Median TTP: 6.5 months
VNB on days 1 and 15 (4-week cycles)		Median OS: 14.5 months
		Four of the 36 patients developed a decline in LVEF (>15%); all four had received prior anthracycline treatment. There were no clinical symptoms of cardiac failure
PLD and VNB	Phase II	ORR: 35%	^Citation57
- 35 mg/m^2 PLD day 1 and 30 mg/m^2		Median TTP: 7 months
VNB day 1 (4-week cycles)		Median OS: 13 months
		Three of the 33 patients had a significant decline in LVEF (<50%); none had clinical cardiac symptoms and cardiac function recovered for two patients after cessation of treatment
EPI/VNB vs PLD/VNB	Phase II	EPI/VNB:	^Citation58
- 90 mg/m^2 EPI day 1 and 25 mg/m^2		ORR: 42.6%
VNB days 1 and 5 (21-day cycle)		Median PFS: 10.7 months
40 mg/m^2 PLD day 1 and 30 mg/m^2		Median OS: 34.6 months
VNB days 1 and 15 (4-week cycles)		PLD/VNB:
		ORR: 52%
		Median PFS: 8.8 months
		Median OS: 24.8 months
		No cases of CHF, two transient LVEF decreases in arm EPI/VNB that resolved in 2 months

Abbreviations: 5-FU, 5-fluorouracil; AUC, area under the concentration curve; CHF, congestive heart failure; DFS, disease-free survival; DOR, duration of response; ECG, electrocardiogram; EPI, epirubicin; GEM, gemcitabine; HER2, human epidermal growth factor receptor 2; L-DOX, liposomal doxorubicin; LVEF, left ventricular ejection fraction; ORR, overall response rate; OS, overall survival; pCR, pathological complete response; pCRT, total pathological complete response; PFS, progression-free survival; PLD, PEGylated L-DOX; TTP, time to progression; VNB, vinorelbine.

Methods

A literature search was conducted on PubMed, Google Scholar, and ClinicalTrials.gov using the following main keywords: liposomal DOX, Doxil^® and BC, Caelyx^® and BC, and Doxil^® or Caelyx^® and combinations in BC, to obtain relevant publications evaluating the safety and efficacy of L-DOX in the treatment of breast tumors. Additionally, information regarding the PK and safety profiles of L-DOX and conventional DOX was acquired mainly from drug monographs. Publications assessed in this review included Phases II and III clinical trials of patients with BC, ranging from early to metastatic stages. No particular preference was given in regard to BC subtype, and the only criteria that needed to be met for combinations were the use of L-DOX in at least one arm with at least one additional agent.

Results

Tables 1 and 2 highlight the disparity in PK parameters and toxicity profiles between L-DOX and DOX. These can be attributed to differences in the formulation of the two agents, with the encapsulation of free DOX into a phospholipid bilayer and exterior PEGylation of the liposomes providing improvements in terms of increasing the drug’s half-life (T _1/2), decreasing both the volume of distribution (V _d) and plasma CL, and reducing the severity of toxicity associated with the use of anthracyclines. L-DOX’s decreased CL (~ 0.7 vs 324–809 mL/ min/m² for DOX) and increased T _1/2 (55±4.8 vs 20–48 h for DOX) may be attributed to decreased metabolism by the liver and MPS. L-DOX liposomes are ~80–90 nm in diameter, although some references state that the size of the molecule is ≥100 nm, a characteristic that impedes their passage across hepatic sinusoidal epithelial fenestrations and decreases their metabolism by hepatocytes. In addition, PEGylation of liposomes decreases their opsonization by immunoglobulin/ complement proteins and their uptake by phagocytic cells of the MPS (eg, Kupffer cells and splenic macrophages), thus prolonging the agent’s plasma circulation time.

Another advantage of L-DOX is its extremely small volume of distribution in comparison to that of DOX (2.72±0.12 vs 809–1,214 L/m²). While DOX’s large V _d indicates that it can effectively distribute into all compartments of the body, its lack of selectivity for tumors means that it can cause a wide range of toxicities. In contrast, the small volume of distribution of L-DOX indicates that the drug is mostly confined into the vascular space, with little free DOX available, as the drug is contained within the liposomes and does not distribute freely to healthy tissues. The small size of L-DOX allows it to extravasate more selectively across fenestrations in the epithelium of blood vessels supplying tumors, where it releases DOX, meaning that generally the use of L-DOX is associated with milder side effects.^5,14 The classic adverse effect associated with DOX use is cardiotoxicity that can range in severity from an acute form that develops shortly after exposure to DOX to a more severe late form where patients may experience decreases in left ventricular ejection fraction (LVEF) and a subsequent diagnosis of DOX induced CHF. Furthermore, the use of DOX is limited by a cumulative lifetime dose limit of up to 550 or 450 mg/m² if a patient received previous mediastinal radiation. In contrast, the more common toxicities from L-DOX use include palmar–plantar erythrodysesthesia, nausea, and alopecia.^16,21–25

Since DOX is considered one of the most effective chemotherapy drugs available, it is often added to regimens for localized or metastatic BC as first- or second-line therapy, as a part of a neoadjuvant therapy prior to surgery or as a salvage therapy. Although it is an effective agent, the risks of cardiotoxicity, particularly when combined with other medications associated with the development of CHF, such as trastuzumab and cyclophosphamide, can limit its use.²⁶ In the case of L-DOX, the decreased rates of cardiotoxicity due to the formulation/PK differences described in the preceding paragraph allows its inclusion in regimens where free DOX would have a high risk of cardiotoxicity. Table 3 summarizes several trials where L-DOX has been combined with other chemotherapeutics or targeted therapies. Of note, a study combining L-DOX, trastuzumab, and cyclophosphamide was one of the most effective, with an overall survival of 34.2 months and the progression-free survival (PFS) of 12 months.²⁷ In terms of toxicity, eight of the 48 included patients experienced asymptomatic decreases in LVEF and all but one recovered; of the affected patients, six patients had prior exposure to anthracyclines. As for the other trials included in Table 3, a majority did not find any significant changes to LVEF or high incidences of clinically relevant cardiotoxicity; however, in several instances where mild-to-moderate cardiotoxicity was reported, it was often in patients who either had prior anthracycline exposure or were concurrently being treated with trastuzumab.^27–31

Discussion

The decreased risk for the cardiotoxicity of L-DOX combined with its comparable efficacy to DOX in the treatment of BC has made it a suitable alternative therapy in treatment regimens that traditionally utilized conventional DOX.¹⁶ In the in vivo setting, the prolonged systemic circulation of L-DOX due to its relatively long half-life,³² along with its selective delivery to the tumor site due to its extravasation through leaky tumor vasculature,³³ results in a higher tumor accumulation as compared to normal tissues. In addition, circulating free-drug concentrations in plasma are reduced due to the highly stable L-DOX formulation, leading to lower cardiac tissue exposure of free-DOX, as compared to tumor tissue. Thus, the use of L-DOX would not only be able to alleviate cardiotoxicity but also to retain significant cytotoxic activity against target tumor cells, due to differences in exposure as well as relative potency of DOX in both tissue types. This is in agreement with results from a Phase III study,¹⁶ where L-DOX was shown to be as efficacious as DOX, with significantly reduced cardiotoxicity and other adverse events, in patients with metastatic BC.

Table 3 summarizes all clinical trials for combinatorial effects of L-DOX with other chemotherapeutics and targeted agents. It is noted that practically all trials are Phase II, and the cardiotoxic events observed were either very low or not existent. In most cases where patients experienced mild-to-moderate cardiotoxicity, they were reported to have received prior anthracycline therapy or were on regimens that included trastuzumab, which is known to augment cardiotoxicity caused due to DOX.³⁴ Nevertheless, the cardiotoxicity observed in the case of L-DOX was significantly lower than that observed with DOX, thus establishing the cardiac safety of this formulation and supporting its clinical use.

In this work, we sought to discuss the therapeutic use of L-DOX in BC. A review of available Phase II and III trials in BC patients has demonstrated that the use of L-DOX generally causes very little cardiotoxicity, while retaining efficacy when used in combination with other chemotherapeutics. Together, this information suggests that L-DOX should continue to be evaluated in further Phase II and III trials in BC, as it remains an effective agent when combined with other chemotherapeutics and is a reasonable agent to substitute in the place of conventional DOX, particularly in patients who are at higher risk for cardiotoxicity.

Disclosure

The authors report no conflicts of interest in this work.