Uniform assessment and ranking of opioid µ receptor binding constants for selected opioid drugs.

The safe disposal of unused opioid drugs is an area of regulatory concern. While toilet flushing is recommended for some drugs to prevent accidental exposure, there is a need for data that can support a more consistent disposal policy based on an assessment of relative risk. For drugs acting at the Mu-opioid receptor (MOR), published measurements of binding affinity (K(i)) are incomplete and inconsistent due to differences in methodology and assay system, leading to a wide range of values for the same drug thus precluding a simple and meaningful relative ranking of drug potency. Experiments were conducted to obtain K(i)'s for 19 approved opioid drugs using a single binding assay in a cell membrane preparation expressing recombinant human MOR. The K(i) values obtained ranged from 0.1380 nM (sufentanil) to 12.486 µM (tramadol). The drugs were separated into three categories based upon their K(i) values: K(i) > 100 nM (tramadol, codeine, meperidine, propoxyphene and pentazocine), K(i)=1-100 nM (hydrocodone, oxycodone, diphenoxylate, alfentanil, methadone, nalbuphine, fentanyl and morphine) and K(i) < 1 nM (butorphanol, levorphanol, oxymorphone, hydromorphone, buprenorphine and sufentanil). These data add to the understanding of the pharmacology of opioid drugs and support the development of a more consistent labeling policies regarding safe disposal.

Metabolic profile of XK469 (2(R)-[4-(7-chloro-2-quinoxalinyl)oxyphenoxy]-propionic acid; NSC698215) in patients and in vitro: low potential for active or toxic metabolites or for drug-drug interactions.

As part of an ongoing phase 1 study, we studied the excretion of XK469 and its metabolism in patients and in vitro. Five primary metabolites were identified by HPLC/MS/MS. An oxidized product formed by cytosolic aldehyde oxidase was the predominant species both in urine and human hepatocytes in vitro. Conjugates of XK469 with glycine, taurine, and glucuronic acid, as well as the microsomal product, 4-oxo-XK469, were also found in urine and in vitro, but none were major contributors to the mass balance for XK469 elimination. Based upon the relative concentrations circulating in plasma, systemic exposure to parent drug was 100-fold higher than for the metabolites. Thus, both toxicity and efficacy of XK469 are most likely to be produced by the parent molecule, rather than the metabolites. Urinary recovery of parent drug was low (2% of dose in 24 h), partly because of the long half-life of XK469 (approximately 3 days). In addition, the metabolite profile in urine indicates that only 25% of the XK469-derived material was unchanged drug. Thus, urinary excretion was not a major factor in XK469 elimination. Variations in systemic exposure to XK469 will be strongly influenced by factors that alter the activity of aldehyde oxidase, including pharmacogenetics, enzyme inhibition, and enzyme induction, but no specific modifiers have been reported. The multiday half-life of XK469 hampered our ability to obtain a complete mass balance, and the possibility exists that other routes, such as biliary excretion, may also play a substantial role in XK469 disposition.

Metabolism by N-acetyltransferase 1 in vitro and in healthy volunteers: a prototype for targeted inhibition.

Inhibition of drug metabolism is generally avoided but can be useful in limited circumstances, such as reducing the formation of toxic metabolites. Acetylation is a major pathway for drug elimination that can also convert substrates into toxic species, including carcinogens. Sulfamethoxazole, a widely used antibiotic, is metabolized via arylamine N-acetyltransferase 1. p-Aminosalicylate, used for antitubercular treatment, is also metabolized by N-acetyltransferase 1 and could potentially inhibit sulfamethoxazole metabolism. Human hepatocytes from 4 donors were incubated in vitro with sulfamethoxazole and paminosalicylate at clinically achievable concentrations. p-Aminosalicylate competitively reduced the acetylation of sulfamethoxazole in vitro by 61% to 83% at 200 microM. Four healthy volunteers were studied following doses of 500 mg sulfamethoxazole either alone or during administration of paminosalicylate (4 g ter in die). Plasma concentrations of paminosalicylate exceeded 100 microM. With each subject as his or her own control, p-aminosalicylate reduced by 5-fold the ratio of plasma concentrations of acetylsulfamethoxazole relative to parent drug (P < .001). Metabolic drug-drug interaction studies in vitro successfully predicted inhibition of acetylation via N-acetyltransferase 1 in vivo. Although no specific toxic species was investigated in this work, the potential was demonstrated for improving the therapeutic index of drugs that have toxic metabolites.