The opiate anomalies--another possible explanation?

Bioactive conformation of linear peptides in solution: an elusive goal?

Bioactive peptides of natural origin have, in general, short linear sequences, and are characterized by a large conformational flexibility. It is very difficult to study their conformation in solution since they exist, almost invariably, as a complex mixture of numerous conformers, most of which are extended. The so-called bioactive conformation may be one of them, although the solvents used in solution studies often have properties drastically different from those of the biological system in which the peptide acts. There is, however, no simple way of identifying the bioactive conformation amid the many existing conformers. It is possible to approach a solution to this problem using two distinct strategies: (a) Limiting the conformational freedom of the peptide, e.g., by increasing the viscosity of the solution and decreasing the temperature, in the assumption that the bioactive conformation is, even slightly, more stable than the others. (b) Trying to mimic in solution the physicochemical features of the more reliable receptor models. These two approaches will be illustrated with examples taken mainly from opioid peptides.

Leucine enkephalin analogues containing a conformationally restrained N-terminal amino acid residue

Three analogues of leucine enkephalin, in which the terminal tyrosine-1 residue has been replaced by conformationally restrained aromatic amino acids, have been synthesized by classical solution methods. Their opiate agonist potencies on electrically stimulated guinea pig ileum and mouse vas deferens preparations were determined and compared with morphine, Met enkephalin, and Leu enkephalin. None of these analogues had analgesic properties when evaluated on the above tissue preparations or when evaluated by the hot-plate test in mice after subcutaneous and intracerebroventricular administration.

Analgesic and other pharmacological activities of a new narcotic antagonist analgesic (-)-1-(3-methyl-2-butenyl)-4-[2-(3-hydroxyphenyl)-1-phenylethyl]-piperazine and its enantiomorph in experimental animals

Of 1-chclohexyl-4-[2-(3-hydroxyphenyl)-1-phenylethyl]piperazine (I) and its 1-(3-methyl-2-butenyl) derivative (II), the S(+)-isomers were analgesically more active than either their +(-)-isomers or their racemates, having 15 to 44 times the potency of morphine in mice and rats. R(-)-I had comparable analgesic activity to morphine R(-)-II to pentazocine in mice, rats and dogs and they were nearly equipotent with pentazocine in reversing some actions of morphine. The S(+)-isomers and racemates lacked this action. R(-)-II required about 10 times more naloxone to reverse its analgesic activity than was needed to antagonise the S(+)-isomers, morphine and pentazocine. The S(+)-isomers and racemates produce a typical Straub tail reaction and increased spontaneous locomotor activity in mice, but the R(-)-isomers did not. R(-)-II had no significant physical dependence liability in mice, rats and monkeys. From these results, it is suggested that the compounds show an uncommon steroselectivity in comparison with morphine and its surrogates, and that R(-)-II is worth investigating further as a narcotic antagonist analgesic.

The natural tolerance of the afghan pika (Ochotona rufescens) to morphine

A lagomorph, the afghan pika, Ochotona rufescens showed no effect whatever following the subcutaneous injection of morphine in doses up to 50 mg per 100 g of body weight, i.e. 250 times the ED50 for the rat. Higher doses were toxic and induced convulsions. However, the pika is responsive to synthetic opiates such as etorphine, pentazocine and phenoperidine. Interestingly enough, morphine antagonized the opiate response elicited by those narcotics to which the animal is sensitive. Pharmacokinetic analysis demonstrated that morphine enters the pika's brain as readily as it does the rat's. In addition, opiate receptor sites, which are present in normal amounts in pika brain retained their high affinity for 3H-etorphine (KD = 0.3 nM), 3H-naloxone (KD = 1.2 nM) and morphine. Moreover, binding of morphine to pika brain homogenates was inhibited in the presence of sodium ions (agonist response). Therefore, the antagonism of phenoperidine action by morphine appeared not to occur at the opiate receptor site; the mechanism of the pika's natural tolerance to morphine may reside in molecular events that normally preceed (metabolism?) or follow (enzyme activation?) the interaction between the drug and its specific recognition sites.

Elucidation of the receptor-bound conformation of the enkephalins

The biologically relevant conformers of enkephalin predicted by solid state, solution state, and theoretical energy studies have been compared with the published structure-activity data on these compounds. No conformational technique proposes a model consistent with all the pharmacological data; the shortcomings of each approach are evaluated. An alternative approach, which correlates the structure-activity data of opiate compounds with that of the enkephalins, is described and shown to produce a model consistent with the available structure-activity data.