Novel disulphide esters of carbothioic acid as potent, non-detergent spermicides with low toxicity to Lactobacillus and HeLa cells in vitro

Abstract

The design, synthesis, characterization and evaluation of a novel series of non-detergent spermicides has led to the discovery of two unique molecules (DSE-36 and DSE-37) that were approximately 25 times more potent spermicides than nonoxynol-9 (N-9). Normal human spermatozoa were used to assess the spermicidal activity (Sander-Cramer Assay), the effect on sperm-membrane integrity [hypo-osmotic swelling test (HOST)], supravital staining and scanning electron microscopy (SEM) and the induction of apoptosis [fluorescein isothiocyanate (FITC) Annexin-V and JC-1 labelling using flow cytometry] by the new class of compounds. HeLa and Lactobacillus cultures were used to assess the cytotoxicity of compounds and their compatibility to normal vaginal flora, respectively. Compounds DSE-36 and DSE-37 exhibited a strong spermicidal activity [minimum effective concentration (MEC) = 0.002%], which was approximately 25 times more potent than that of N-9 and Sapindus saponins (MEC = 0.05%). As compared with surfactants, DSE-36 and DSE-37 were found to be safer at MEC towards the growth and survival of Lactobacilli and HeLa cells in vitro and to have a milder effect on sperm plasma membrane. At EC(50) both induced apoptosis in sperm cells as characterized by increased labelling with Annexin-V and decreased polarization of sperm mitochondria. Preliminary studies have revealed that in sharp contrast to the non-specific surfactant action of N-9, DSE-36 and DSE-37 have a highly potent, mechanism-based, detrimental action on human sperm. The unique ability of these non-detergent molecules to selectively kill sperm and spare Lactobacilli and HeLa cells at MEC values much lower than that required for N-9 indicates their potential as superior ingredients for formulation into microbicidal contraceptives.