In vitro testing of rationally designed spermicides for selectively targeting human sperm in vagina to ensure safe contraception

BACKGROUND: Rational synthesis of novel structures resulted in two unique molecules (DSE-36 and DSE-37, disulphide esters of carbothioic acid) that killed sperm 25 times more strongly and with a precisely targeted action than nonoxynol-9 (N-9). We examine the effects of DSE-36 and DSE-37 on human spermatozoa versus HeLa cells to establish specificity and safety compared with N-9. METHODS AND RESULTS: At spermicidal EC I00 (20 μg/ml) DSE-36 and DSE-37 killed 100% sperm in <30 s (Sander-Cramer assay) and at EC 50 induced apoptosis in sperm (Annexin-V-fluorescein isothiocyanate and JC-I labelling and Flow Cytometry) in 3 h. However, at EC I00 these molecules had no effect on HeLa cells by 24 h or on cell viability [3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) assay], surface ultrastructure (scanning electron microscopy), Annexin-V and JC- I labelling pattern and reactive oxygen species (ROS) generation. N-9, with a spermicidal EC I00 of 500 μg/ml, decreased HeLa cell viability at 20 μg/ml in 24 h (P < 0.001), accompanied by acute damage to cell surface ultrastructural topography, induction of apoptosis and ROS generation. Unlike DSE-36 and DSE-37, N-9 also significantly induced mRNA levels (RT-PCR) of pro-inflammatory biomarkers (interleukin (IL)-1α, IL-6, IL-8, RANTES) in HeLa cells and increased IL-6 and IL-8 secretion (P < 0.001, enzyme-linked immunosorbent assay). Furthermore, DSE-36 and DSE-37 did not inhibit Lactobacillus growth at EC I00 and exhibited mild microbicidal activity against Trichomonas vaginalis, while N-9 inhibited Lactobacillus and Trichomonas growth but had a lower prophylactic index. CONCLUSIONS: The ability of these novel spermicides to kill sperm almost instantaneously at innocuously low concentration indicates their worth as improved active ingredients for vaginal contraceptive preparations compared with N-9.