Pharmacokinetics and CSF entry of flurazepam in dogs

Lipophilicity at pH 7.4 and molecular size govern the entry of the free serum fraction of drugs into the cerebrospinal fluid in humans with uninflamed meninges

Physicochemical properties of drugs were related to their ability to enter the cerebrospinal fluid (CSF) in humans by reevaluation of previously reported studies. Either the quotients of the drug concentrations in CSF and serum at steady state (CCSFSS/CSSS) or, since in most cases CSF passage was studied after a short-term infusion, the ratios of the areas under the concentration-time curves in CSF and serum (AUCCSF/AUCS) were taken as measures of CSF passage. AUCS and CSSS were corrected for binding to serum proteins (AUCSf, CSssf). Of the drugs studied the quotient of the octanol/water partition coefficient at pH 7.4 (PC) as a measure of lipophilicity and the square root of the molecular weight (MW1/2) correlated with AUCCSF/AUCS (Spearman's rank correlation coefficient rS = 0.78, P < 0.01) and with AUCCSF/AUCSf (rS = 0.90, P < 0.01). PC.MW-1/2 was related to AUCCSF/AUCSf (or CCSFSS/CSssf, respectively) by the equation: AUCCSF/AUCSf = 0.96 + 0.091.1n(PC.MW-1/2). For 0.0001 < or = PC.MW-1/2 < or = 1.0 this function may be of value for the prediction of CSF penetration in humans when the physicochemical properties of a drug are known and when active transport or metabolism are negligible.

Pharmacokinetic quantification of the exchange of drugs between blood and cerebrospinal fluid in man

Various parameters which may be useful in quantification of drug transit from blood into CSF and vice versa after a short duration infusion are compared here by recalculating previously published data from our group. Due to the slower entry into and elimination from the CSF compartment as compared to the central compartment, the ratio of drug concentrations in CSF and serum sampled at the same time increase with time after an infusion. Therefore, concentration quotients of simultaneously drawn blood and CSF are inadequate to characterise CSF penetration. The ratio of the areas under the concentration-time curves in a body fluid and serum (AUCbody fluid/AUCs) is an established measure to quantify overall penetration from the central into a peripheral compartment. AUCCSF/AUCs is closely correlated with the quotient of the maximum CSF and serum concentrations (CmaxCSF/CmaxS) (rs = 0.87, n = 42, P < 0.001) and with the rate constant of distribution in CSF (CLin/VCSF) (rs = 0.80, n = 42, P < 0.001). Since CmaxCSF/CmaxS depends on the mode of drug administration, it is suggested that AUCCSF/AUCs be used to quantify overall drug transit into CSF. CLin/VCSF is of use when CSF can only be sampled once, or when the velocity of the transit of a drug into CSF is to be described. The CSF exit rate constant (CLout/VCSF) characterises elimination from CSF independent of the elimination from serum and may be applied to estimate the formation rate of CSF; in the present study it averaged 20 ml/h.

Pharmacokinetics and preliminary observations of behavioral changes following administration of midazolam to dogs

The pharmacokinetics of midazolam were investigated following intravenous and intramuscular administration of 0.5 mg of midazolam hydrochloride/kg of body weight to five healthy mixed-breed dogs. One dog also received the same dose of midazolam by oral and rectal routes. The disposition of midazolam following intravenous administration was characterized by very rapid and relatively extensive distribution followed by rapid elimination. Mean (+/- SD) apparent volume of distribution was 3.0 +/- 0.9 l/kg, mean elimination half-life was 77 +/- 18 min, and clearance was 27 +/- 3 ml/kg/min. Following intramuscular administration, absorption was rapid and complete. A mean peak midazolam concentration of 549 +/- 121 ng/ml was reached within 15 min, and systemic availability was over 90% in each dog. Oral administration to one dog resulted in peak midazolam concentrations within 10 min and a systemic availability of 69%. Rectal administration to the same dog yielded very low systemic availability. Midazolam was extensively bound to canine plasma proteins, with the unbound fraction representing less than 4% of the total plasma midazolam concentration. Plasma samples were also assayed for the presence of the major metabolites, 1-OH and 4-OH midazolam. Neither metabolite were detected, probably as a result of rapid elimination of these compounds by hepatic glucuronidation. Behavioral responses to administration of midazolam included initial signs of profound weakness, ataxia and transient agitation followed by a period of quiesence. A normal behavior pattern returned within 2 h of midazolam administration.