Effect of diphenhydramine on methaqualone metabolism: an in vitro study.

A GLC assay to quantitate the methaqualone metabolite 2-methyl-3-(2-hydroxymethylphenyl)-4(3H)-quinazolinone was developed. Standard curves were linear, and recovery of the metabolite from tissue homogenates averaged 89%. In vitro metabolism of methaqualone by the 10,000Xg supernatant fraction of rat liver homogenate was measured by monitoring metabolite formation with the GLC assay. Diphenhydramine inhibited the in vitro metabolism of methaqualone. The percentage inhibition increased with increasing diphenhydramine concentration. The significance of this inhibition in relation to use and abuse of methaqualone--diphenhydramine combinations is discussed.