Discovery of substituted isoxazolecarbaldehydes as potent spermicides, acrosin inhibitors and mild anti-fungal agents

Background: The continued endeavour to design novel, non-detergent molecules that can be useful as topical, prophylactic contraceptives has led to the discovery of substituted isoxazolecarbaldehydes as a new class of compounds exhibiting both spermicidal and acrosin inhibitory activities simultaneously. Methods: Normal human semen samples were used to detect the spermicidal and acrosin inhibitory activities of the new compounds. Lactobacillus, HeLa and Candida cultures were used to determine the safety of compounds towards normal vaginal flora, their cytotoxicity and anti-fungal activity. Supravital staining and the hypo-osmotic swelling test (HOST) were used to detect the effect on sperm membrane integrity. Nonoxynol-9 (N-9) was used as a reference standard. Results: The 5- and 3-substituted isoxazolecarbaldehydes showed significant spermicidal [minimum effective concentration (MEC)=0.005-2.5%] and acrosin inhibitory (IC₅₀ =3.9-58×10⁴mol/l) activities in several molecules along with weak fungicidal activity against Candida albicans Lineweaver-Burk and Dixon plot analysis of a representative structure showed non-competitive inhibition of human acrosin enzyme, and the most potent acrosin inhibitors also considerably diminished the induction of the acrosome reaction by Ca²⁺ ionophore. Some compounds were found to be significantly safer than N-9 towards Lactobacillus acidophilus in vitro at their respective spermicidal MECs. In the cytotoxicity assay, the IC₅₀ of these compounds towards the HeLa cell line was of the same order as N-9 (0.9-0.1 mmol/l); however, in contrast, the compounds exhibited only a moderate effect on sperm membrane integrity. Conclusions: This study indicates that 5- and 3-substituted isoxazolecarbaldehydes are 'first generation' multifunctional, spermicidal molecules that hold promise for development as topical contraceptives with useful associated activities that can add considerably to their effectiveness, safety and prophylaxis.