Correlation of opiate receptor affinity with analgetic effects of meperidine homologues

The affinity for opiate receptor sites in brain tissue in a series of N-substituted meperidine homologues has been compared with the analgetic potency of these compounds in mice. There is a good correlation between affinity for opiate receptor binding sites assayed in the presence of sodium and analgetic potency for homologues whose N-substituent has six or fewer carbons. The apparent discrepancy between the weak affinity of these drugs for opiate receptors and their fairly potent analgetic effects in vivo can be explained by meperidine's efficient penetration into brain.