Buprenorphine - Physicochemical Properties

Physicochemical Properties

Buprenorphine is a semi-synthetic derivative of thebaine, one of the chemically most reactive morphine alkaloids. Buprenorphine has a molecular weight of 467 and its structure is typically opioid with the inclusion of a C-7 side-chain containing a t-butyl group. This group confers overall lipophilicity on the molecule which has an important influence on its pharmacology.

Opioids exert their pharmacological effects by binding to opioid receptors. The pharmacological effects are determined by the nature of opioid-receptor interaction. Some of these effects such as analgesia, mediated by an agonistic action at the μ-opioid receptor are desirable, whereas others such as nausea, sedation, or constipation can be considered as unwanted adverse effects. Buprenorphine is a μ-opioid receptor agonist with high affinity, but low intrinsic activity. Compared with morphine which behaves as a full μ-opioid agonist, buprenorphine is usually defined as a partial μ-opioid agonist that shows high affinity for and slow dissociation from the μ-opioid receptor. A full dose-dependent effect on analgesia has been seen within the clinically relevant dose range (up to 10 mg), but no respiratory depression which levels off at higher doses (Dahan et al. 2005). Clinically, there is also a less marked effect of buprenorphine-binding to μ-opioid receptors on gastrointestinal transit times, and indeed constipation seen in the clinic is remarkably low (Griessinger et al. 2005). Buprenorphine also shows partial agonistic activity at the opioid receptor-like receptor 1 (ORL1)-receptors which are (at least at supraspinal receptors) postulated to induce a pronociceptive effect. A study by Lutfy et al. (2003) reported that co-activation of ORL1-receptors compromises the antinociception induced by activation of the μ-opioid receptor. ORL1-activation has also an effect on hyperalgesia. It might be that buprenorphine’s partial agonism reduces this effect compared with full ORL1-agonists such as morphine or fentanyl. Buprenorphine’s antagonistic action at the δ-receptors which have a marked anti-opioid action and seem to negatively modulate central analgesia seems further to contribute to its clinically seen analgesic effect. Its likewise antagonistic activity at the κ-opioid receptors might explain the fact that it induces much less sedation and psychotomimetic effects than morphine or fentanyl (Lewis 1985; Leander 1988). Animal studies have shown that buprenorphine has a 20–40 times higher potency than morphine (Martin et al. 1976).

The strong binding of buprenorphine to the μ-opioid receptor has several consequences. Initial binding is relatively slow compared with other opioids such as fentanyl (Boas and Villiger 1985). However, the onset of analgesia is not dissimilar, since buprenorphine achieves effective analgesia at relatively low receptor occupancy (5%–10%) (Tyers 1980) and thus relatively low plasma concentrations of buprenorphine are sufficient to provide effective pain relief. The slow dissociation of buprenorphine from the receptor results in a long duration of effect and also confers another advantage in that when the drug is withdrawn an abstinence syndrome is rarely seen because of the long time taken for the drug to come off the receptor (Bickel et al. 1988)