Comparative studies of the pharmacological effects of the d- and l-isomers of codeine

Opiates are known for their stereospecificity. The following studies show that l-codeine was active in the mouse tail-flick test as well as in the hot plate test whether given p.o. or s.c. The ED50 in the first test was 4.09 mg/kg s.c. (2.01-8.34 mg/ kg) and 13.41 mg/kg p.o. (6.91-26.0 mg/kg). In the second antinociceptive test, the ED50 was 20.66 mg/kg s.c. (11.52-37.08 mg/kg) and 20.47 mg/kg p.o. (14.63-28.57 mg/kg). The d-isomer of codeine was inactive ina both tests up to 100 mg/kg but caused hyperexcitability, convulsions and ultimately death. Although l-codeine was more potent than d-codeine inhibiting the cough reflex in the anesthetized cat, the d-compound did have good activity. The ED50 of the l-isomer was 0.27 mg/kgi.v. (0.14-0.47 mg/kg) and that of the d-isomer was 1.61 mg/kg i.v. (0.98-2.65 mg/kg). In these animals, l-codeine did not significantly affect the cardiovascular parameters at the doses tested, whereas d-codeine caused a significant but transient decrease in the blood pressure and heart rate. The specific and nonspecific properties of d- and l-codeine were further delineated in the opiate receptor binding assay. l-Codeine inhibited the stereospecific binding of 2.2 x 10(-9) M [3H]dihydromorphine in mouse brain homogenate with the IC50 being 1.6 x 10(-5) M (1.2 x 10(-5)--2.0 x 10(-5) M). d-Codeine had no effect up to 10(-4) M.