These data demonstrate that the in vitro and in vivo pharmacological profile of meloxicam is structurally dependent and that minor structural changes can lead to significant differences in the selectivity for COX-1 and COX-2 in vitro and to different profiles in vivo suggesting different therapeutic potential.

This study determined if meloxicam, a selective cyclooxygenase (COX)-2 inhibitor, interferes with the antiplatelet effect of aspirin using platelet aggregation and thromboxane (Tx) B(2) endpoints in healthy volunteers. Eight male and 8 female volunteers participated in this open-label, randomized, two-treatment, two-way crossover trial. Treatment 1 was meloxicam (15 mg qd) over 4 days, and then aspirin (100 mg qd) was ingested 2 hours after meloxicam for an additional 7 days. Blood samples were taken 2, 6, and 24 hours after the last dose. Treatment 2 consisted of only aspirin (100 mg) over 2 days. Samples were taken at the same time points. Each subject received both treatments with a 2-week washout between the treatment periods. Treatments were safe and well tolerated. The initial 4-day treatment with meloxicam had no effect on platelet aggregation but reduced serum TxB(2) by 64% +/- 19%. Addition of aspirin (100 mg qd) for 7 days resulted in complete inhibition of aggregation and TxB(2) for 24 hours. Two-day treatment with only 100 mg aspirin also resulted in complete inhibition of platelet aggregation and TxB(2). These results indicate that meloxicam does not affect the ability of aspirin to inhibit COX-1 in platelets, thereby allowing aspirin to effectively prevent platelet aggregation and reduce TxB(2) levels, and that meloxicam is selective for COX-2.

1. The metabolic fate of 14C-labelled meloxicam was investigated in the urine and bile of rat following oral and intraduodenal administration. Structural elucidation of metabolites was performed by nuclear magnetic resonance, mass spectrometry (electron impact and fast atom bombardment). 2. A mean total of 76.3% 14C-radioactivity was recovered in urine over 96 h, with the remainder in the faeces. The metabolic pattern in the excreta was independent of dose (1 versus 10 mg/kg) and collection period (0-8 versus 24-48 h). In bile one of the main metabolites was absent. 3. Meloxicam underwent extensive metabolism with only small amounts of unchanged drug recovered in the urine (< 0.5%) or bile (4.5%). Principal routes of biotransformation were: oxidation of the 5-methyl group of the N-heteroaryl-carbamoyl side chain to yield the 5'-hydroxymethyl derivative (33% of metabolites in urine, 22% in bile) and the 5'-carboxy derivative (16% in urine, 49% in bile). Oxidative cleavage of the benzothiazine-ring yielded an oxamic acid metabolite in urine (23.5%), which was not present in bile. 4. The introduction of a methyl-group into the N-heteroaryl-carbamoyl side chain increased lipophilicity and facilitated metabolic excretion compared with structurally related compounds.

In order to gain some insight into those structural features that control M1 selectivity, a selected set of pirenzepine analogues has been studied in which both the tricyclic ring system and the basic side chain have been varied. Binding studies were conducted in rat tissue homogenates from cerebral cortex (M1) and gastric fundus (M2). The ratio of IC50 values of the test compounds in the two different tissues was taken as a measure of M1 receptor selectivity. Several derivatives, especially those with flexible side chains, i.e. high degree of freedom of rotation around single bonds, proved to be nonselective. Among semirigid compounds only those containing 6-membered ring systems (11, 13, 14, and 15) showed significant M1 selectivity. Principles of structure-activity and structure-selectivity are discussed.

On the basis of the cardioselective muscarinic receptor antagonist AF-DX 116 (2), a series of 11-substituted pyridobenzodiazepinones (9-35) was prepared and screened for their binding affinity to muscarinic receptors located in cardiac (M2) and glandular (M3) tissue. The ratio of IC50 values of the test compounds in the two different tissues was taken as a measure of cardiac (M2) receptor selectivity. Qualitative structure-selectivity relationships point to the fact that it is the spatial orientation of the protonated side-chain nitrogen atom in relation to the tricycle that is the main determinant for receptor subtype recognition and hence is important for the achievement of cardiac (M2) selectivity.