Oxymorphone is the 14 beta-hydroxyl version of hydromor-phone, analogous to the hydrocodone, oxycodone pair discussed above. Although the addition of the 14 beta-hydroxyl group to hydrocodone (30 mg) yielded oxycodone (20 mg), a more potent drug, the opposite is true for the conversion of hydromorphone (7.5 mg) to oxymorphone (10 mg). The reason for this is that the oral bioavailability of oxymorphone (10%) is lower than that of hydromorphone (35%) because of decreased absorption and increased first-pass metabolism. Presumably, the addition of the OH group does increase its binding affinity at the receptor as the injectable form of oxymorphone (1 mg) is more potent than injectable hydromorphone (1.5 mg).
Oxymorphone is available as a suppository (5 mg), an injection (1 mg/mL), an immediate-release tablet (5 mg, 10 mg), and in 2003 the FDA approved a sustained release formulation (Opana ER 5 mg, 10 mg, 20 mg, and 40 mg). The 12-hour coverage of the extended release tablet provides another option for those patients suffering from chronic pain. The side effect profile of the extended release formulations of morphine, oxycodone, and oxymorphone are similar, and there appears to be no clear advantage of one over the other.
The morphinans were made by removing the E ring of morphine, the 4,5-ether bridge, in an attempt to simplify the structure (Fig. 24.7).
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