Pharmacokinetics of flunitrazepam in rats studied by a radioreceptor assay

Fundamental aspects of radioreceptor assays

Significant advances in understanding the actions of hormones, neurotransmitters and drugs resulted from the development of new analytical procedures. Among these advances the discovery that, for a given drug (or hormone or neurotransmitter), there exists a receptor site on membranes to which the substance must attach in order to evoke a biological response, led to the setting up of the radioreceptor assays. Thus ligand-binding techniques can be used as assays for measuring drug levels in biological tissues and fluids. Radioreceptor assays are based on the principle of competitive protein-binding methods; when a radioligand and an unlabelled ligand are present together with a specific receptor preparation, the amount of radioactive ligand bound to the receptor is a quantitative function of the amount of unlabelled ligand present in the incubation medium. The technology of the radioreceptor assays is simple and rapid to perform when optimal conditions for binding are determined. Radioreceptor assays are highly sensitive, reliable, precise and accurate. Their chief limitation relates to specificity as any substance having an appreciable affinity for the receptor displaces the specifically bound radioligand. Paradoxically this lack of specificity may be in some cases advantageous in that it allows for the detection of the parent compound and active metabolites in proportion to their affinity for the specific receptor. During recent years, radioreceptor assays have been applied to quantitative determination of hormones, neurotransmitters and drugs. Among these, the radioreceptor assay of benzodiazepines is a typical example. In addition to drug analysis and drug monitoring, radioreceptor assays have been found to be versatile tools for basic studies on the receptor itself, on the endogenous ligand(s) for the receptor and on the discovery and the identification of biologically active chemical entities (pharmacological screening). The future of radioreceptor assays in the pharmacological sciences appears to be promising.

Flunitrazepam variably alters morphine, buprenorphine, and methadone lethality in the rat

Opiates and substitution products are frequently abused, alone and in association with benzodiazepines. While this combination may result in severe respiratory depression and death, the quantitative relationship remains uncertain. We performed randomized, blinded intravenous median lethal dose (MLD) studies in Sprague-Dawley rats of morphine, buprenorphine, and methadone, alone and in combination with intraperitoneal flunitrazepam pretreatment. We employed the up-and-down method, performed in quadruplicate, comparing time to death following opioid injection. Results are expressed as median of four series (extremes). The MLDs of morphine, buprenorphine, and methadone alone were 64.0 (33.6:79.5), 234.6 (168.6:284.4), and 22.5 (19.3:24.1) mg/kg, respectively, and 60.6 (35.2:88.2), 38.4 (30.6:54.0), and 13.0 (9.7:13.8) mg/kg, respectively, after pretreatment with 40 mg/kg flunitrazepam. Times to death for morphine, buprenorphine, and methadone alone were 2.5 (0.8:24), 0.02 (0.0:24), and 2.0 (0.0:24) hours, respectively, and 13.5 (0.0:144), 24.0 (0.0:120), and 0.0 (0.0:24) hours, respectively, after pretreatment with flunitrazepam 40 mg/kg, ip. Flunitrazepam significantly altered methadone (P=0.02) and buprenorphine (P=0.02) but not morphine lethality (P=0.77). Flunitrazepam significantly prolonged time to death only for buprenorphine (P<0.01). Flunitrazepam-opioid drug-drug interactions are more complex than is generally believed. Mechanistic studies of flunitrazepam-opioid lethal interactions are needed.

Flunitrazepam and nordiazepam slowly released from silastic capsules induce physical dependence in rat

The rates of in vitro release of flunitrazepam (FN), nordiazepam (ND) and diazepam (DZ) from silastic capsules were compared and found to be in the following order: DZ > FN > ND. Rats that were implanted subcutaneously with capsules filled with FN or ND for 5 to 7 weeks before administering flumazenil (FLU) (40 mg/kg, i.v.) showed precipitated abstinence as measured by the Precipitated Abstinence Score (PAS) which included a rapid onset of clonic and tonic-clonic convulsions. Rats implanted with DZ also demonstrated significant PAS and seizures. Implantation of similar doses of DZ, FN and ND resulted in different plasma levels of parent benzodiazepines and their metabolites that corresponded with their in vitro release: DZ > FN > ND. These data indicate that, as for DZ, the capsule implantation is an effective method of producing physical dependence on FN and ND in the rat.

Pharmacokinetic differences of desmethyldiazepam in three outbred Wistar strains related to differences in liver enzyme activities

Various strains of rats differ in their sensitivities towards benzodiazepines suggesting that variations in biological features are responsible for these differences. All reports concern inbred strains. We have studied male rats of three outbred Wistar strains with regard to the pharmacokinetics of desmethyldiazepam given as a single intravenous dose. The elimination half-life of desmethyldiazepam was longer in the Riv:TOX(M) (from the National Institute of Public Health and Environmental Protection) and in the Cpb:WU strain than in the Crl:(WI)BR strain. This strain difference in elimination of desmethyldiazepam may be related to differences in liver enzyme activities.

Relationship between receptor occupancy at 37 degrees C and the anticonvulsant effect of flunitrazepam in rats

In this investigation an attempt was made to evaluate quantitatively the relationship between benzodiazepine receptor occupancy and the anticonvulsant effect of flunitrazepam in rats. A graded measure of anticonvulsant effect was obtained on the basis of an elevation of pentylenetetrazol (PTZ) threshold concentrations. The concentration-anticonvulsant effect relationship could be described by the Emax model with an EC50 in cerebrospinal fluid of 2.9 +/- 0.8 micrograms/liter and an Emax of 227 +/- 22 mg/liter PTZ (mean +/- SE). In vitro receptor occupancy was determined in a crude brain homogenate at 0 and 37 degrees C, which yielded KD values of 2.2 +/- 0.2 and 26 +/- 2 micrograms/liter, respectively. The results obtained in both experiments were combined by focusing on free flunitrazepam concentrations. This strategy resulted in a nonlinear relationship between receptor occupancy and anticonvulsant effect of flunitrazepam, with 90% of the maximum response achieved at a degree of receptor occupancy of approximately 50% at 37 degrees C.