Stability of 18 at different pH. Compound 18 was incubated in 0.1 M HCl (pH 1) and phosphate-buffered saline (PBS, pH 7.5), aliquoted at different intervals, and analyzed by HPLC. Peak area was calculated from HPLC chromatogram. Plot percentage of compound remaining vs. time in days for A) pH 1, B) pH 7.5. Compound 18 indicated degradation at pH 1 but was stable for 30 days in PBS at pH 7.5. In vitro stability of 18 in the presence of human liver microsomes. C) Compound 18 was incubated with human liver microsomes, extracted at particular time points and analyzed by HPLC. A plot of relative concentration of 18 with respect to time is shown. For standardization of the assay verapamil was used. D) Mass spectra of sample at 0 (left) and 6 h (right) incubated in liver microsomes.