Immunomodulators: use in combined therapy against leishmaniasis

Host immune responses have a direct impact on the efficacy of chemotherapy against leishmaniasis, one of the most important of the neglected tropical diseases. Although there have been significant advances in recent years to minimize drug toxicity, cost and treatment duration, notably with the development of short-course combined therapy for visceral leishmaniasis, the potential for drug resistance and a paucity of new drugs in development remain major challenges for the future. The use of immunomodulators in combination with conventional chemotherapy to enhance host immune responses may have several advantages as a means to improving current therapeutic regimens.

Leishmaniasis

There are over 20 species of the protozoan parasite Leishmania that are transmitted by sandflies and can cause human disease. Leishmaniasis presents as a range of clinical manifestations from self-healing localized skin ulcers (cutaneous leishmaniasis [CL]) to severe, life-threatening systemic disease (visceral leishmaniasis [VL] or kala-azar) [1]. CL is usually self-limiting but can become disseminated or undergo metastatic spread to the mucosae (mucocutaneous leishmaniasis), forms of disease that do not heal. Post kala-azar dermal leishmaniasis, with widespread cutaneous lesions, is an increasingly common sequelae of VL [2].

Visceral leishmaniasis is caused by Leishmania donovani and Leishmania infantum, and is largely confined to East Africa, the Indian subcontinent, Brazil and regions bordering the Mediterranean. There are an estimated 500,000 new cases and 70,000 deaths reported for VL per annum, although these figures may represent a significant underestimate [3,4]. Hallmarks of VL include hepatosplenomegaly and disruption of lymphoid tissue microarchitecture. These are features linked to immunosuppression and are likely to underlie VL-associated secondary infection [5].

Conventional chemotherapy

For the last 60 years, first-line therapy for leishmaniasis was limited to parenteral administration of antimonial compounds (Pentostam^® [GlaxoSmithKline, Middlesex, UK]; Glucantime^® [Sanofi-Aventis, Paris, France]) and this is still the most common treatment of CL, disseminated CL and mucocutaneous CL. However, antimony resistance amongst parasites causing VL has increased to an extent that in the major endemic focus of Bihar, India, antimonial drugs have limited value, being superseded by intravenous amphotericin B deoxycholate and, more recently, paromomycin [6,7]. Nevertheless, all these drugs require long treatment regimens (21–30 days), have significant toxicity, and require intensive clinical and laboratory monitoring. In Bihar, short-course liposomal amphotericin B (AmBisome^® [Gilead Sciences, Cambridge, UK]) therapy has proved effective for VL, and a recent study found a single infusion of AmBisome to be as effective as 15 infusions of amphotericin B deoxycholate. Preferential pricing for use in developing countries has increased the likelihood of broad usage of this drug [8]. Miltefosine, the only oral drug for VL, has limited use due to concerns about drug resistance, patient compliance and its teratogenic potential [4]. Combined approaches to therapy are increasingly being recognized as the way forward in the treatment of leishmaniasis [9,10], with good supporting evidence of efficacy from early clinical trials. However, should resistance to the limited number of available drugs appear, it could lead to the rapid loss of therapeutic options. The potential use of drugs that target the host rather than the parasite represents an alternative strategy for combined therapy. The recognition that many anti-leishmanial drugs operate in synergy with host immune mechanisms has fuelled interest in developing combined immunochemotherapy [11,12].

Combined immunochemotherapy

Synergy between chemotherapy and host immune function was first suggested by observations that immunocompromised patients with VL failed to respond to antimonial drugs [13]. Studies in experimental models of VL confirmed that the efficacy of pentavalent antimony (Sb^v) was T-cell dependent [14]. This observation initiated a slew of studies aimed at either bypassing the T-cell requirement by direct provision of T-cell-derived effector cytokines (cytokine therapy), or by augmenting endogenous T-cell activation through enhanced antigen presentation (Toll-like receptor [TLR]-based and costimulation-based therapy). In immunodeficient mice, the T-cell-derived cytokines IL-2 and IFN-γ restore the activity of Sb^v[14]. Treatment with recombinant (r)IFN-γ reduced the amount taken to produce an effective dose in 50% of the participants of Sb^v tenfold. Sb^v is believed to be a prodrug requiring intracellular conversion to Sb^III for leishmanicidal activity, therefore, the increased uptake of the drug by IFN-γ-activated macrophages has been proposed as a mechanism underlying this treatment response [15]. There has been some success in clinical trials with combined therapy of IFN-γ and Sb^v. IFN-γ treatment accelerated responses and enhanced Sb^v efficacy; additionally, Sb^v-refractory patients retreated with Sb^v plus IFN-γ also showed long-term responses to treatment [16,17]. More recently, IL-12, a cytokine that potentiates T-cell IFN-γ production, was shown to have similar effects in both CL [18] and VL in mice [11,19].

To potentiate T-cell activation, successful immunotherapy has been achieved through activating antigen-presenting cells with TLR agonists. Imiquimod, used topically for the treatment of genital warts, activates TLR7 and -8 on antigen-presenting cells, and mediates the production of a variety of pro-immune cytokines, including IL-12 and IFN-γ [20,21]. Treatment of CL patients in a recent Phase III clinical trial with imiquimod and antimony showed a higher cure rate (75%) compared to that seen in patients treated with placebo and antimony (58%) [22]. GM-CSF has also been successfully employed in the clinic [23], though whether this beneficial effect was due to reversal of neutropenia or enhanced recruitment of antigen-presenting cells is not clear.

A fusion protein that stimulates T cells through OX40, as well as a agonist monoclonal antibody (mAb) against CD40, promoted host protective immunity and supported low-dose Sb^v therapy in mice [24,25]. Blockade of cytotoxic T-lymphocyte associated (CTLA)-4, a negative regulator of T-cell costimulation using either mAb [26] or anticalin therapy [27] has a beneficial effect in experimental VL, and anti-CTLA-4 also showed significant synergy with Sb^v[24]. Most recently, a receptor tyrosine kinase inhibitor (sunitinib maleate), used clinically in the treatment of renal cell carcinoma [28,29], was shown to be able to restore lymphoid tissue microarchitecture in an experimental model of VL, and to provide a tenfold dose-sparing effect when used in a pretreatment regimen with Sb^v[30].

In VL, each of the previously described approaches must overcome the strong immunosuppressive environment of the infected individual. Hence, not surprisingly, the alternative approach of removing endogenous suppression has also received attention. In VL, cytokine-mediated immunosuppression is dominated by IL-10 and TGFβ [31,32]. IL-10 is known to promote intracellular infection by limiting IL-12 and IFN-γ secretion, costimulation, and IFN-γ-induced macrophage activation. Hence IL-10-deficient mice are highly resistant to VL [5,32]. This cytokine also impairs responsiveness to Sb^v[14]. In experimental models of VL, treatment with mAb against the IL-10 receptor allowed a 35-fold reduction in the effective dose of Sb^v compared with drug alone, as well as considerably shortening the time for effective therapy [32,33]. Inhibition of TGF-β has been shown to decrease parasite burdens in experimental VL; however, TGF-β blockade had no apparent effect on Sb^v activity [34].

Challenges

While the pharmacokinetics of drug action and the nature of drug–immune interactions in mouse and human may differ, studies in animal models can provide new clues to potential approaches, and the active pursuit of further clinical trials of combined immune chemotherapy for leishmaniasis is clearly needed. However, immunomodulation based on mAbs remains largely focused on diseases in developed countries and medium-to-small-scale target populations amenable to advanced patient care. While such highly profitability markets remain the primary focus for drug development, it seems unlikely that such products will find their way into clinical trials for neglected tropical diseases such as leishmaniasis. Increased access to drug libraries may provide a means for access to small-molecule drugs, such as the receptor tyrosine kinase inhibitors, though considerable resource will still be required to select best-fit drugs for application to the leishmaniasies. Removal of endogenous immunosuppressive cytokines has obvious potential, but also poses the risk of unleashing an overwhelming proinflammatory response. Finally, the increasing efficacy of current combined therapy with anti-leishmanial drugs may itself be a hindrance to new drug development, significantly increasing the complexity of trials needed to show additional benefit.

Financial & competing interests disclosure

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.

No writing assistance was utilized in the production of this manuscript.