Determinants of benzodiazepine brain uptake: lipophilicity versus binding affinity

Abstract

Factors influencing brain uptake of benzodiazepine derivatives were evaluated in adult Sprague Dawley rats (n = 8-10 per drug). Animals received single intraperitoneal doses of alprazolam, triazolam, lorazepam, flunitrazepam, diazepam, midazolam, desmethyldiazepam, or clobazam. Concentrations of each drug (and metabolites) in whole brain and serum 1 h after dosage were determined by gas chromatography. Serum free fraction was measured by equilibrium dialysis. In vitro binding affinity (apparent Ki) of each compound was estimated based on displacement of tritiated flunitrazepam in washed membrane preparations from rat cerebral cortex. Lipid solubility of each benzodiazepine was estimated using the reverse-phase liquid chromatographic (HPLC) retention index at physiologic pH. There was no significant relation between brain:total serum concentration ratio and either HPLC retention (r = 0.18) or binding Ki (r = -0.34). Correction of uptake ratios for free as opposed to total serum concentration yielded a highly significant correlation with HPLC retention (r = 0.78, P less than 0.005). However, even the corrected ratio was not correlated with binding Ki (r = -0.22). Thus a benzodiazepine's capacity to diffuse from systemic blood into brain tissue is much more closely associated with the physicochemical property of lipid solubility than with specific affinity. Unbound rather than total serum or plasma concentration most accurately reflects the quantity of drug available for diffusion.

Mechanism-Based Pharmacokinetic/Pharmacodynamic Modeling of the Electroencephalogram Effects of GABAAReceptor Modulators: In Vitro-in Vivo Correlations

“…Correction for the free fractions in plasma resulted in a statistically significant increase of the correlation coefficient (p Ͻ 0.001), indicating that free plasma concentrations may be the most representative for concentrations at the site of action and the in vivo drug effect. Similar observations have been reported for benzodiazepines (Greenblatt et al, 1983;Arendt et al, 1987;Mandema and Danhof, 1992;Hoogerkamp et al, 1996) and neuroactive steroids (Visser et al, 2002a). For DMCM no free fraction could be determined in the ultrafiltrate despite a sufficiently low detection limit of the assay and might indicate a methodological error (e.g., sticking of DMCM to the membrane of the ultrafiltration device) or that protein binding is not an important factor for the pharmacodynamics of DMCM.…”

Relationship Between Exposure and Nonspecific Binding of Thirty-Three Central Nervous System Drugs in Mice

“…As such, the mouse and rat brain-to-plasma ratios predicted by these unbound fractions are very similar at 1.69 Ϯ 0.08 and 1.63 Ϯ 0.05, respectively. However, the actual in vivo B/P estimates of midazolam in mice and rats are approximately 10-fold different at 0.23 (Table 2) and 2.3 (Arendt et al, 1987;Mandema et al, 1992). These data together indicate that the A, represents the degree to which B/P was over-or under-predicted by fu plasma /fu brain ratio.…”

Effects of alprazolam and lorazepam on catecholaminergic and cardiovascular activity during supine rest, mental load and orthostatic challenge

“…The plasma concentration curves after 1 mg ALP and 2 mg LOR were very similar with respect to the time course during the experiment and with respect to absolute values. Radioreceptor assays have shown lorazepam to be about three times as potent as alprazolam at the central benzodiazepine receptor, but alprazolam's free fraction in serum is 2-3 times higher than lorazepam's (Arendt et al 1987). This does not make it easy to estimate the concentrations and activity of both compounds at the central benzodiazepine receptor when serum concentrations are similar.…”