Mechanism of Action

ULTRAM ER is a centrally acting synthetic opioid analgesic. Although its mode of action is not completely understood, from animal tests, at least two complementary mechanisms appear applicable: binding of parent and M1 metabolite to µ-opioid receptors and weak inhibition of reuptake of norepinephrine and serotonin.

Opioid activity is due to both low affinity binding of the parent compound and higher affinity binding of the O-demethylated metabolite M1 to µ-opioid receptors. In animal models, M1 is up to 6 times more potent than tramadol in producing analgesia and 200 times more potent in µ-opioid binding. Tramadol-induced analgesia is only partially antagonized by the opiate antagonist naloxone in several animal tests. The relative contribution of both tramadol and M1 to human analgesia is dependent upon the plasma concentrations of each compound.

Tramadol has been shown to inhibit reuptake of norepinephrine and serotonin in vitro, as have some other opioid analgesics. These mechanisms may contribute independently to the overall analgesic profile of tramadol. The relationship between exposure of tramadol and M1 and efficacy has not been evaluated in the ULTRAM ER clinical studies.

Apart from analgesia, tramadol administration may produce a constellation of symptoms (including dizziness, somnolence, nausea, constipation, sweating and pruritus) similar to that of other opioids. In contrast to morphine, tramadol has not been shown to cause histamine release. At therapeutic doses, tramadol has no effect on heart rate, left- ventricular function or cardiac index. Orthostatic hypotension has been observed.

Pharmacokinetics

The analgesic activity of tramadol is due to both parent drug and the M1 metabolite. ULTRAM ER is administered as a racemate and both the [-] and [+] forms of both tramadol and M1 are detected in the circulation.

The pharmacokinetics of ULTRAM ER are approximately dose-proportional over a 100-400 mg dose range in healthy subjects. The observed tramadol AUC values for the 400-mg dose were 26% higher than predicted based on the AUC values for the 200-mg dose. The clinical significance of this finding has not been studied and is not known.

Absorption

In healthy subjects, the bioavailability of a ULTRAM ER 200 mg tablet relative to a 50 mg every six hours dosing regimen of the immediate-release dosage form (ULTRAM) was approximately 85-90%. Consistent with the extended-release nature of the formulation, there is a lag time in drug absorption following ULTRAM ER administration. The mean peak plasma concentrations of tramadol and M1 after administration of ULTRAM ER tablets to healthy volunteers are attained at about 12 h and 15 h, respectively, after dosing (See Table 1 and Figure 2). Following administration of the ULTRAM ER, steady-state plasma concentrations of both tramadol and M1 are achieved within four days with once daily dosing.

The mean (%CV) pharmacokinetic parameter values for ULTRAM ER 200 mg administered once daily and tramadol HCl immediate-release (ULTRAM) 50 mg administered every six hours are provided in Table 1.

Table 1: Mean (%CV) Steady-State Pharmacokinetic Parameter Values (n=32)

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