Lofentanil

Lofentanil is an opioid analgesic that is an analogue of fentanyl and was developed in the early 1980s.

Lofentanil is most similar to the highly potent opioid carfentanil (4-carbomethoxyfentanyl). Lofentanil can be described as 3-methylcarfentanil, or 3-methyl-4-carbomethoxyfentanyl. However it is interesting to note that while 3-methylfentanyl is considerably more potent than fentanyl itself, lofentanil is only slightly stronger than carfentanil. This suggests that substitution at both the 3 and 4 positions of the piperidine ring introduces steric hindrance which prevents μ-opioid affinity from increasing much further. As with other 3-substituted fentanyl derivatives such as ohmefentanyl, the stereoisomerism of lofentanil is very important, with some stereoisomers being much more potent than others.

Lofentanil is very similar to carfentanil in effects, but has a longer duration of action. This makes it unsuitable for most practical applications, with carfentanil being the preferred agent for tranquilizing large animals, and short-acting derivatives such as sufentanil or remifentanil being preferred for medical use in human surgical procedures. The long duration and high lipophilicity of lofentanil has been suggested as an advantage for certain types of analgesia, but the main application for lofentanil at the present time is research into opiate receptors. Side effects from lofentanil would be predicted to be similar to other potent fentanyl analogues, and would include sedation, euphoria, nausea, and pronounced itching and respiratory depression. Side effects from lofentanil might be particularly problematic given its reportedly long duration of action. Another side effect which is characteristic of fentanyl and its derivatives is their tendency to rapidly induce tolerance, due to their high binding affinity triggering rapid internalization of chronically activated opiate receptors. This might be expected to be a particular problem with lofentanil as it is not only one of the most potent drugs in the series, but also is longer acting than most other fentanyl analogues, meaning that development of tolerance triggered by receptor over-activation is likely to be especially rapid.