Stability of carmethizole hydrochloride (NSC-602668), an experimental cytotoxic agent

The solution stability and degradation mechanism of Carmethizole HCl (1, NSC-602668), an experimental anti-cancer drug, were studied. An apparent pK_a value of 4.2 at 25°C and ionic strength = 0.15 M (NaCl) was determined for 1 by potentiometric titration. The degradation 1 in aqueous solution proceeds by two consecutive hydrolysis reactions of the bis-carbamate moieties at C4′ and C5′, with hydrolysis at C5′ hydrolyzing approximately 8 times faster than hydrolysis at C4′. The rates of degradation of 1 in aqueous solution were determined as a function of pH, temperature, buffer concentration, ionic strength, and co-solvent concentration. The pH of maximum stability was at pH < 2 for the degradation of 1 at 25°C. Analysis of the hydrolysis products by electron impact mass spectrometry showed that one ¹⁸O atom was incorporated into the first degradation product, 2, when the reaction was run in ¹⁸O-enriched water. This is consistent with alkyl C-O bond cleavage rather than an addition-elimination reaction at a carbamate group. The entropy of activation (ΔS^‡) was -6.71 ± 0.05 e.u.at pH 9.90, indicating a unimolecular mechanism. All observations were consistent with an SN1 mechanism.