Structure/activity relationships of a homologous series of surfactants (nonyl-phenoxypolyethoxyethanols) on rat vaginal bioelectric activity over the oestrous cycle.

The spermicide nonoxynol-9 is a member of a homologous series of alkylphenol-ethoxylates (polyethoxyethanols) of general formula C9H19-C6H6-O-(CH2CH2O)n-1 CH2CH2OH. Nonoxynol-9 when instilled into the vagina of anaesthetized rats causes a rapid fall in the transvaginal potential difference (p.d.). Using the time taken for the p.d. to fall to half its value (t 1/2), the structure/activity relationship of a number of isosmolar solutions of the compounds on the vaginal p.d. was assessed during the oestrous cycle. Members of the series tested had n = 1.5, 4, 6, 8/9, 9, 10/11, 15, 20, 30 and 40 and were commercially available (Antarox CO series, GAF). At pro-oestrus all compounds were ineffective. At metoestrus and oestrus the relationship between the activity of the series and the n value was clearly U-shaped, a similar pattern was apparent at dioestrus. When n = 40, 30 or 20 little or no effect was observed on the p.d. but the depressive activity was significant at n = 15 and became maximal at n = 8/9 and 9. When n = 6 the activity began to attenuate while at n = 4 and 1.5 it became very reduced. The relation between the ability to depress the p.d. and the moles of ethylene oxide (n) appeared to be related to the hydrophilic/hydrophobic balance of the compound. When n was greater than 15 the compounds were too hydrophilic to enter the membrane phase and cause disruption while when n less than 6 they were too hydrophobic and had too small a molar volume to affect membrane function.(ABSTRACT TRUNCATED AT 250 WORDS)