Immobilized Artificial Membrane HPLC Derived Parameters vs PAMPA-BBB Data in Estimating in Situ Measured Blood-Brain Barrier Permeation of Drugs

The affinity indexes for phospholipids (log kWIAM) for 42 compounds were measured by High Performance Liquid Chromatography (HPLC) on two different phospholipid-based stationary phases (Immobilized Artificial Membrane - IAM). The polar/electrostatic interaction forces between analytes and membrane phospholipids (Δlog kWIAM) were calculated as the differences between the experimental values of log kWIAM and those expected for isolipophilic neutral compounds having Polar Surface Area (PSA) = 0. The values of Blood-Brain barrier (BBB) passage measured in situ, P0in situ, for 38 analytes were taken from the literature, representing the permeability of the neutral forms on “efflux minimized” rodent models. Furthermore, the values of passage through a porcine brain lipid extract (PBLE) artificial membrane for 36 analytes, measured by so-called PAMPA-BBB technique, were taken into account (P0PAMPA-BBB). A total of 42 compounds were considered, being 32 analytes in common to both sets. The values of log P0PAMPA-BBB were found significantly related to the n-octanol lipophilicity of the neutral forms (log PN) whereas poor relationships were found with lipophilicity of the mixtures of ionized and neutral forms existing at the experimental pH 7.4 (log D7.4). Furthermore no relationship was found with either log kWIAM or Δlog kWIAM values. The values of log P0in situ related moderately to log P0PAMPA-BBB (32 data points) but did not relate with either n-octanol lipophilicity indexes (log PN and log D7.4) or phospholipid affinity indexes (log kWIAM). In contrast, significant inverse linear relationships were observed between log P0in situ (38 data points) and Δlog kWIAM values for all compounds but ibuprofen and chlorpromazine who behaved as moderate outliers. It should be remembered that BBB passage, parameterized by log BB values, i.e. the logarithm of the ratio brain concentration/ blood concentration, was previously found linearly related to Δlog kWIAM for bases, ampholytes, and neutral compounds whereas for acids the measure of polar/electrostatic interactions required the correction for ionization giving a different parameter, Δ’log kWIAM. However, the result of the present work are not in contrast with the results previously found because log P0in situ refer to the “intrinsic permeability” of the analytes regardless their ionization degree. Furthermore, log BB values and log P0in situ are roughly linearly related for all the analytes but those predominantly present at the experimental pH 7.4 as anions. The latter are included in the relationship only if log P0in situ values corrected for ionization (log PCin situ) are taken into account. These results suggest that ΔlogkWIAM parameter is effective at predicting log P0in situ values for all the analytes and that ionization affects much more markedly BBB passage of acids (yielding anions) than that of the other ionisable compounds.