Mechanism of action

Tramadol acts as a μ-opioid receptor agonist,^[50][51] serotonin releasing agent,^{[3][4][52][53]} norepinephrine reuptake inhibitor,^[51] NMDA receptor antagonist,^[54] 5-HT_2C receptor antagonist,^[55] (α7)₅ nicotinic acetylcholine receptor antagonist,^[56] TRPV1 receptor agonist,^[57] and M₁ and M₃ muscarinic acetylcholine receptor antagonist.^[58][59]
The analgesic action of tramadol has yet to be fully understood, but it is believed to work through modulation of serotonin and norepinephrine in addition to its mild agonism of the μ-opioid receptor. The contribution of non-opioid activity is demonstrated by the fact that the analgesic effect of tramadol is not fully antagonised by the μ-opioid receptor antagonist naloxone.
Tramadol is marketed as a racemic mixture of the (1R,2R)- and (1S,2S)-enantiomers with a weak affinity for the μ-opioid receptor (approximately 1/6000th that of morphine; Gutstein & Akil, 2006). The (1R,2R)-(+)-enantiomer is approximately four times more potent than the (1S,2S)-(–)-enantiomer in terms of μ-opioid receptor affinity and 5-HT reuptake, whereas the (1S,2S)-(–)-enantiomer is responsible for noradrenaline reuptake effects (Shipton, 2000). These actions appear to produce a synergistic analgesic effect, with (1R,2R)-(+)-tramadol exhibiting 10-fold higher analgesic activity than (1S,2S)-(–)-tramadol (Goeringer et al., 1997).
The serotonergic-modulating properties of tramadol give it the potential to interact with other serotonergic agents. There is an increased risk of serotonin toxicity when tramadol is taken in combination with serotonin reuptake inhibitors (e.g., SSRIs), since these agents not only potentiate the effect of 5-HT but also inhibit tramadol metabolism.^{[citation needed]} Tramadol is also thought to have some NMDA antagonistic effects, which has given it a potential application in neuropathic pain states.
Tramadol has inhibitory actions on the 5-HT_2C receptor. Antagonism of 5-HT_2C could be partially responsible for tramadol's reducing effect on depressive and obsessive-compulsive symptoms in patients with pain and co-morbid neurological illnesses.^[60] 5-HT_2C blockade may also account for its lowering of the seizure threshold, as 5-HT_2C knockout mice display significantly increased vulnerability to epileptic seizures, sometimes resulting in spontaneous death. However, the reduction of seizure threshold could be attributed to tramadol's putative inhibition of GABA-A receptors at high doses.^[54]
The overall analgesic profile of tramadol supports intermediate pain, especially chronic states. It is slightly less effective for acute pain than hydrocodone, but more effective than codeine. It has a dosage ceiling similar to codeine, a risk of seizures when overdosed, and a relatively long half-life making its potential for misuse relatively low amongst intermediate strength analgesics.
Tramadol's primary active metabolite, O-desmethyltramadol, is a considerably more potent μ-opioid receptor agonist than tramadol itself, and is so much more so that tramadol can partially be thought of as a prodrug to O-desmethyltramadol. Similarly to tramadol, O-desmethyltramadol has also been shown to be a norepinephrine reuptake inhibitor, 5-HT_2C receptor antagonist, and M₁ and M₃ muscarinic acetylcholine receptor antagonist.^{[citation needed]}

[edit] Chemistry

[edit] Characteristics

Structurally, tramadol closely resembles a stripped down version of codeine. Both codeine and tramadol share the 3-methyl ether group, and both compounds are metabolized along the same hepatic pathway and mechanism to the stronger opioid, phenol agonist analogs. For codeine, this is morphine, and for tramadol, it is the O-desmethyltramadol.

[edit] Comparison with related substances

Structurally, tapentadol is the closest chemical relative of tramadol in clinical use. Tapentadol is also an opioid, but unlike both tramadol and venlafaxine, tapentadol represents only one stereoisomer and is the weaker of the two, in terms of opioid effect. Both tramadol and venlafaxine are racemic mixtures. Structurally, tapentadol also differs from tramadol in being a phenol, and not an ether. Also, both tramadol and venlafaxine incorporate a cyclohexyl moiety, attached directly to the aromatic, while tapentadol lacks this feature. In reality, the closest structural chemical entity to tapentadol in clinical use is the over-the-counter drug phenylephrine. Both share a meta phenol, attached to a straight chain hydrocarbon. In both cases, the hydrocarbon terminates in an amine.

[edit] Synthesis and stereoisomerism

 
(1R,2R)-Tramadol     (1S,2S)-Tramadol
 
(1R,2S)-Tramadol     (1S,2R)-Tramadol
The chemical synthesis of tramadol is described in the literature.^[61] Tramadol [2-(dimethylaminomethyl)-1-(3-methoxyphenyl)cyclohexanol] has two stereogenic centers at the cyclohexane ring. Thus, 2-(dimethylaminomethyl)-1-(3-methoxyphenyl)cyclohexanol may exist in four different configurational forms:

• (1R,2R)-isomer
• (1S,2S)-isomer
• (1R,2S)-isomer
• (1S,2R)-isomer

The synthetic pathway leads to the racemate (1:1 mixture) of (1R,2R)-isomer and the (1S,2S)-isomer as the main products. Minor amounts of the racemic mixture of the (1R,2S)-isomer and the (1S,2R)-isomer are formed as well. The isolation of the (1R,2R)-isomer and the (1S,2S)-isomer from the diastereomeric minor racemate [(1R,2S)-isomer and (1S,2R)-isomer] is realized by the recrystallization of the hydrochlorides. The drug tramadol is a racemate of the hydrochlorides of the (1R,2R)-(+)- and the (1S,2S)-(–)-enantiomers. The resolution of the racemate [(1R,2R)-(+)-isomer / (1S,2S)-(–)-isomer] was described^[62] employing (R)-(–)- or (S)-(+)-mandelic acid. This process does not find industrial application, since tramadol is used as a racemate, despite known different physiological effects ^[63] of the (1R,2R)- and (1S,2S)-isomers, because the racemate showed higher analgesic activity than either enantiomer in animals^[64] and in humans.^[6