Optimisation of a Topical Miltefosine Carbopol-Based Gel for Cutaneous Leishmaniasis: Antileishmanial Activity After Storage and Drug Response

We previously demonstrated the therapeutic potential of a 0.5% miltefosine carbopol-based gel (G-MTF) in experimental cutaneous leishmaniasis (CL). To support formulation optimisation before clinical evaluation, this study assessed whether G-MTF retained antileishmanial activity after storage at different temperatures and durations and evaluated its dose–response following topical application to CL lesions in BALB/c mice infected with Leishmania amazonensis. Storage effects on some physicochemical and morphological properties, sterility, and MTF detection were also assessed. In vitro antileishmanial activity was largely maintained, although prolonged storage at 35–45 °C moderately reduced potency. During 60 days of follow-up, CL lesions reactivated in 17% of the 118 treated mice, with most reactivations occurring after treatment with G-MTF formulations stored for 3–6 months at 35–45 °C. Dose–response analysis showed concentration-dependent effects on lesion reduction and parasite burden, with estimated ED50 values of 3.64 and 4.02 mg/kg, respectively. All carbopol-based gel formulations remained sterile and retained physicochemical characteristics at 8–28 °C, with pH values within the normal skin range. Storage at 8 °C increased viscosity and reduced spreadability, whereas higher temperatures increased swelling. MALDI-TOF MS demonstrated that MTF remained detectable after prolonged storage, while SEM showed preserved morphology at 8–28 °C. Overall, carbopol-based G-MTF maintained therapeutic activity and acceptable physicochemical characteristics for up to six months at 8–28 °C. Validated quantitative stability-indicating and drug-release methods are needed to confirm MTF chemical stability and drug availability in the carbopol-based gel.