A Neglected Disease Under Therapeutic Pressure

Visceral leishmaniasis (VL), caused by Leishmania donovani and Leishmania infantum, generates an estimated 200,000–400,000 new cases annually, with mortality concentrated in impoverished populations across South Asia, East Africa, and South America. The disease is macrophage-tropic: amastigotes persist within phagocytic cells of the liver, spleen, and bone marrow, evading host immunity by modulating cytokine signalling and exploiting drug efflux mechanisms. Left untreated, VL is potentially fatal.

The therapeutic landscape is shaped by three converging pressures. First, resistance: laboratory studies confirm that L. donovani can develop resistance to established drug combinations — including AmB/paromomycin and miltefosine/paromomycin — via step-wise adaptation. Second, toxicity: the nephrotoxicity of conventional amphotericin B deoxycholate and the teratogenicity of miltefosine restrict their use in vulnerable populations. Third, access: liposomal amphotericin B, while highly effective, carries cost and cold-chain barriers that limit deployment at primary healthcare level in endemic regions.

Molecular target identification has accelerated across several fronts, as catalogued in retrieved academic literature. The parasite’s obligate requirement for ergosterol — absent in mammalian cells — underpins the mechanism of amphotericin B and azole-class drugs. Beyond ergosterol, researchers have identified druggable kinases (CRK12), the proteasome, mRNA-processing factors (CPSF), parasite tubulin, and trypanothione reductase as tractable targets with preclinical validation. Elevated TGF-β levels in drug-resistant VL and parasite-induced PD-1 upregulation on T cells have additionally framed immunological vulnerabilities as therapeutic entry points, according to WHO-classified neglected tropical disease research programmes.

LAmB: From Monotherapy to Combination Regimens

Liposomal amphotericin B (AmBisome®) is the most extensively evidenced treatment modality in the VL literature, with a total dose of 20 mg/kg confirmed effective in immunocompetent patients across geographic settings. The University of Geneva review reports LAmB efficacy data in more than 8,000 VL patients treated by Médecins Sans Frontières in South Asia, demonstrating safety at primary healthcare level — while noting that access and cost remain significant barriers and that generic LAmB versions are in development.

“LAmB at 20 mg/kg total dose is effective in immunocompetent VL patients across geographic settings — yet HIV/VL co-infected patients require dedicated randomised trials to optimise dosing, and cost barriers persist in the highest-burden regions.”

The combination frontier is most advanced in post-kala-azar dermal leishmaniasis. A Phase II open-label randomised trial by DNDi evaluated paromomycin/miltefosine/LAmB triple combinations for PKDL in Sudan, representing the most clinically advanced combination evidence in this dataset. Separately, a comparative randomised open-label study in India directly assessed amphotericin B lipid emulsion (ABLE) versus LAmB in 500 VL patients (3:1 ratio), providing head-to-head formulation data relevant to cost-efficacy trade-offs.

Pharmacokinetic and pharmacodynamic modelling of AmBisome and miltefosine in a BALB/c L. donovani model, conducted by the London School of Hygiene & Tropical Medicine, supports dose-optimisation for combination scheduling. A critical translational risk, however, is documented by the Dundee/IPBLN study: L. donovani can develop resistance to AmB/paromomycin and miltefosine/paromomycin combinations via step-wise adaptation in laboratory conditions — a finding that underscores the urgency of identifying mechanistically distinct combination partners.

The Indian Institute of Chemical Biology has additionally characterised KALSOME™10, a liposomal formulation whose ergosterol-rich composition is described as mechanistically relevant to leishmanicidal efficacy, with both therapeutic and immunomodulatory activities documented in retrieved results. According to WHO guidelines on neglected tropical diseases, combination regimens are prioritised to reduce treatment duration, lower individual drug doses, and delay resistance emergence.

Nanoparticle Delivery Systems Reshaping AmB Pharmacology

Nanoformulation strategies for amphotericin B address three limitations of conventional LAmB: cost, systemic toxicity, and poor oral bioavailability. Retrieved academic literature documents at least eight distinct nanoparticle platforms at preclinical stage, spanning polymeric, lipidic, and conjugate architectures, each exploiting different biological targeting mechanisms.

Mannose-modified PLGA nanoparticles, developed by the Laboratory of Nanomedicine, enhance in vitro and in vivo VL efficacy versus unmodified formulations by exploiting mannose receptor-mediated macrophage uptake — directly targeting the intracellular niche where amastigotes reside. PEGylated liposomes from the Federal University of Minas Gerais produced 100–130 nm vesicles with greater than 95% encapsulation efficiency, comparable to AmBisome in murine cutaneous leishmaniasis models.

Two platforms specifically address oral delivery — the most clinically important unmet need. Banaras Hindu University developed hydroxypropyl-β-cyclodextrin (HPCD)-modified solid lipid nanoparticles co-encapsulating AmB and paromomycin, achieving 96% and 90% entrapment efficiency respectively with a 141 nm particle size — enabling oral combination delivery in murine VL. The University of Navarra’s nanostructured lipid carrier (NLC) system separately rendered a class IV (BCS) trypanothione reductase inhibitor orally bioavailable, with spleen and liver parasite burden reduction in L. infantum-infected BALB/c mice. According to NIH-funded research on neglected tropical disease drug delivery, oral formulations are a priority for field-deployable VL treatment in endemic settings.

Two additional platforms address selectivity and single-dose convenience. AmB nanodisks from the Children’s Hospital Oakland Research Institute demonstrated marked therapeutic efficacy at 2 mg/kg IV in L. major-infected mice. Implantable PLGA microparticles (0.5–20 µm) from the Federal University of Rio de Janeiro achieved a selectivity index of 50 versus free AmB’s selectivity index of 25, enabling intralesional single-dose delivery. The University of Dundee evaluated HPMA polymer-drug conjugates combining AmB with alendronic acid, finding no in vitro or in vivo advantage for the combination over the mono-conjugate — an instructive negative result for conjugate design strategies.

Novel Molecular Targets and Repurposed Compounds

The most advanced novel small-molecule candidates in the VL pipeline target parasite-specific vulnerabilities with no mammalian orthologues, reducing the risk of host toxicity. Three mechanistically distinct preclinical candidates from GlaxoSmithKline/University of Dundee and DNDi are particularly well-evidenced in retrieved literature.

Targeted New Chemical Entities

DNDI-6148 (benzoxaborole): DNDi’s preclinical candidate targets cleavage and polyadenylation specificity factor (CPSF), an mRNA processing enzyme. It achieves greater than 98% parasite burden reduction in vivo with oral bioavailability — a combination of potency and oral route that addresses the two most critical unmet needs in VL therapy simultaneously.

DDD853651/GSK3186899 (pyrazolopyrimidine, CRK12 inhibitor): GlaxoSmithKline and the University of Dundee identified Leishmania CRK12 (cdc-2-related kinase 12) as a druggable parasite kinase target, with this pyrazolopyrimidine-based candidate demonstrating in vivo VL efficacy at preclinical candidate stage.

GSK3494245/DDD01305143 (proteasome inhibitor): The University of Dundee/Wellcome Centre for Anti-Infectives Research described this candidate, discovered through repurposing screens against Trypanosoma cruzi, as demonstrating murine VL efficacy comparable to reference drugs — illustrating the value of cross-parasite target identification.

Repurposed Compounds

Drug repurposing offers accelerated timelines by leveraging existing safety data. Delamanid, an anti-tuberculosis nitroimidazole, achieves sterile cures in murine VL at 30 mg/kg twice-daily for 5 days, mediated by parasite nitroreductase-distinct metabolism — a mechanism entirely separate from its TB activity. Spironolactone, a potassium-sparing diuretic, was identified as the most promising candidate in a 50-drug screening panel at Instituto Oswaldo Cruz/FIOCRUZ, with activity against both L. amazonensis and L. infantum in murine models. Amodiaquine, an antimalarial, was formulated into oral hydroxypropylmethylcellulose microparticles and tested in L. donovani murine VL models by the University of Ghana School of Pharmacy.

Natural product leads add further diversity. Ursolic acid (UA) was evaluated across three separate retrieved papers: as a standalone antileishmanial agent in VL and cutaneous leishmaniasis, as a combination partner potentiating AmB at sub-therapeutic doses of 0.2–1.0 mg/kg AmB with 1.0 mg/kg UA in L. infantum-infected hamsters, and in a 266 nm nanostructured lipid carrier formulation (UA-NLC) with demonstrated VL efficacy. According to DNDi‘s portfolio strategy, natural product-derived leads with demonstrated in vivo efficacy and tractable formulation pathways are prioritised for further development in neglected tropical diseases.

The diarylsulfonamide class warrants attention for its scale of synthesis effort: Centro de Investigaciones Biológicas Margarita Salas (CSIC, Madrid) synthesised 350 compounds, identifying 16 active against intracellular L. infantum amastigotes — a hit rate exceeding 10% — with 5 compounds showing potency comparable or superior to reference drugs. These target Leishmania tubulin at the colchicine-binding domain, exploiting sequence differences between parasite and host alpha/beta tubulin.

Host-Directed Therapies and Immunotherapeutic Approaches

Host-directed therapies (HDTs) represent a conceptually distinct strategy: rather than killing the parasite directly, they modify the host environment to restore immune clearance or remove the cellular sanctuary in which parasites persist. Multiple retrieved results support a growing HDT paradigm across three mechanistic categories.

mTOR Inhibition

Western University Canada reports that rapamycin and GSK-2126458 — but not KU-0063794 — dramatically reduced L. major footpad infection and parasite burden in BALB/c mice at clinically relevant doses. The selectivity among mTOR inhibitors indicates that specific pathway nodes, rather than broad mTOR suppression, determine efficacy — a distinction with direct implications for repurposing existing oncology drugs as VL adjuncts.

Immune Checkpoint Blockade

Parasite-induced upregulation of PD-1 on CD4+ and CD8+ T cells creates a state of T-cell exhaustion that enables persistent infection. Instituto de Biofísica Carlos Chagas Filho (UFRJ) demonstrates that checkpoint blockade with anti-PD-1 and anti-PD-L1 monoclonal antibodies increased IFN-γ-producing CD4+ and CD8+ T cells and reduced parasite burden in L. amazonensis-infected BALB/c mice. Elevated TGF-β levels in drug-resistant VL, documented in retrieved results from Sudan, frame cytokine modulation as a further therapeutic vulnerability in refractory disease.

Macrophage-Targeted and Combinatorial HDT Approaches

Naloxonazine, identified by the University of California San Francisco, acts specifically against intracellular (but not axenic) L. donovani by upregulating vATPases in host macrophages — a mechanism that is host-encoded rather than parasite-encoded, offering a distinct resistance profile. The University of North Carolina at Chapel Hill applied modified resazurin assays to assess combinatorial interactions between HDTs and conventional VL drugs including amphotericin B, providing an in vitro assay framework for co-administration studies. This combinatorial assessment infrastructure is essential for translating HDT candidates into clinical regimens alongside pathogen-directed drugs, as noted by DNDi in its neglected tropical disease research strategy.

“Naloxonazine acts specifically against intracellular — but not axenic — Leishmania donovani by upregulating vATPases in host macrophages, demonstrating that host-encoded mechanisms can be therapeutically targeted without directly engaging the parasite.”

The institutional landscape for this research is geographically distributed. India-based institutions — including Banaras Hindu University, the Indian Institute of Chemical Biology (IICB, Kolkata), and ICMR-affiliated centres — account for the highest volume of retrieved records, reflecting the country’s status as the primary VL-endemic region in South Asia. UK and European drug discovery institutions, notably the University of Dundee/Wellcome Centre for Anti-Infectives Research and GlaxoSmithKline, contribute the most advanced targeted small-molecule candidates. Latin American institutions including FIOCRUZ and the Federal University of Minas Gerais contribute repurposing and natural product leads. According to WHO‘s 2021–2030 roadmap for neglected tropical diseases, multi-partner international collaboration of exactly this type is designated as a core delivery mechanism for VL elimination targets.

A critical structural barrier to cure — identified by the Forsyth Institute — is the CD271+/ABCG2+ bone marrow mesenchymal stem cell niche, which may enable L. infantum to escape drug action including LAmB through ABCG2-mediated efflux. This finding suggests that sterilising cure may require either nanoformulations capable of penetrating this sanctuary or HDT strategies that disrupt the niche itself — connecting the nanoparticle delivery and immunotherapy workstreams into a shared strategic imperative. Researchers at institutions including Nature-published groups have highlighted the BM-MSC niche as a priority area for mechanistic investigation in drug-resistant VL.