Sensitive and specific radioimmunoassays for opiates using commercially available materials. I: Methods for the determinations of morphine and hydromorphone.

Sensitive and specific radioimmunoassay (RIA) methods for the analysis of morphine and hydromorphone in human plasma samples using commercially available materials were developed. The limit of quantitation was 0.3 ng/mL of plasma for morphine and 50 pg/mL of plasma for hydromorphone. An extraction step preceding the RIA quantitatively removed morphine, leaving 99% morphine-3-glucuronide (M3G) and 95% hydromorphone-3-glucuronide (H3G) in the aqueous phase. The specificity of the morphine RIA method for the analysis of clinical samples was confirmed by HPLC quantitation. The antiserum used in the hydromorphone RIA method cross reacted slightly with H3G at 0.66%. However, analysis of clinical samples using the direct versus the extraction RIA showed that the extraction step was necessary for the specific determination of hydromorphone in pharmacokinetic studies. After extraction, only 0.033% of the H3G present in plasma samples would be observed as interference for the free hydromorphone. The RIA methods were shown to be accurate and reproducible with almost 100% recovery of morphine and hydromorphone. They offer convenient alternatives to chromatographic methods for pharmacokinetic studies.