Clinical pharmacokinetics of doxorubicin in combination with GF120918, a potent inhibitor of MDR1 P-glycoprotein.

Previous clinical investigations with doxorubicin indicated that modulators of P-glycoprotein dramatically decrease the systemic clearance of the drug, which complicates the interpretation of toxicity and response data. In the present study, we examined the pharmacokinetics of doxorubicin and GF120918, a novel potent P-glycoprotein inhibitor, in cancer patients in a search for more selective modulation of multidrug resistance (MDR). Seven cohorts (46 patients) received sequential treatments with doxorubicin alone by a 5 min i.v. bolus (50-75 mg/m2), oral GF120918 alone (50 mg q.d.-400 mg b.i.d.), and the combination of doxorubicin and GF120918. Serial blood and urine samples were taken during both treatment courses and analyzed for doxorubicin and its metabolite doxorubicinol by a liquid chromatographic assay. The pharmacokinetic characteristics of doxorubicin in the presence or absence of GF120918 indicate a very minor overall effect of the modulator, except at the highest combined dose level (i.e. 75 mg/m2 plus 400 mg b.i.d.). A limited number of patients experienced significantly increased exposure to doxorubicinol upon combined treatment, which was associated with concomitantly higher plasma levels of GF120918. Sigmoidal maximum-effect models revealed significant correlations (p<0.02) between the area under the curve of doxorubicinol and the percent decrease in neutrophils and platelets. Sigmoidicity factors in the fitted Hill equation were similar between both treatment courses, suggesting no pharmacodynamic potentiation of doxorubicinol myelotoxicity by GF120918. Our data indicate that GF120918 at the tested doses of combination treatment achieves plasma concentrations that reverse MDR in experimental models and it lacks the significant kinetic interaction with doxorubicin observed previously with other modulators. Hence, it may be possible in future trials to assess the contribution of a potent inhibitor of P-glycoprotein activity to the toxicity and activity of doxorubicin with the knowledge that profound plasma pharmacokinetic interactions are unlikely.

Effects of cholesterol on acyl chain dynamics in multilamellar vesicles of various phosphatidylcholines.

Phase modulation fluorescence spectroscopy was used to investigate the influence of cholesterol (0 to 50 mol%) on acyl chain dynamics in multilamellar vesicles of phosphatidylcholine. Four different phosphatidylcholines (DPPC, DOPC, POPC, and egg PC) and six different fluorescent probes (diphenylhexatriene and five anthroyloxy fatty acids) were employed. We found that: (1) Increased cholesterol content had only slight effects on fluorescence lifetimes of the six probes. (2) Increased cholesterol content increased the steady-state fluorescence anisotropy (r) of all the probes except 16-anthroyloxy palmitate (16-AP) in each of the four phosphatidylcholines. (3) Added cholesterol tended to limit the extent of probe rotation (as reflected by r infinity, the infinite-time anisotropy) to a much greater extent than it altered the rate of probe rotation. (4) The tendency for cholesterol to order the structure of the bilayer was greatest in the proximal half of the acyl chains and diminished toward the center of the bilayer. (5) In some phosphatidylcholines the rotations rates of probes located near the bilayer center (diphenylhexatriene and 16-AP) were apparently increased by increasing levels of cholesterol. (6) In several respects dipalmitoylphosphatidylcholine (DPPC) vesicles responded differently to increased cholesterol than vesicles of the other three phosphatidylcholines. (7) A single second-order equation described the relationship between r infinity and r for the five anthroyloxy fatty acid probes in the four different phosphatidylcholines over a wide range of cholesterol content. The data for diphenylhexatriene in the different phosphatidylcholines could not be fit by a single equation.

Determination of fluorescence polarization of membrane probes in intact erythrocytes. Possible scattering artifacts.

The anisotropy of the fluorescence of diphenylhexatriene has been reported to be less in the membranes of intact erythrocytes than in erythrocyte ghost membranes or in membranes prepared from erythrocyte lipids. Evidence is presented that this may be an artifact due to the intense light scattering by the intact erythrocytes.

Effects of anesthetic alcohols on membrane transport processes in human erythrocytes.

1. Anesthetic alcohols (pentanol, hexanol and heptanol) were found to increase the fluidity of red cell membrane lipids as monitored by the fluorescence depolarization of diphenylhexatriene. The relative potency of the alcohols was found to be parallel to their relative membrane/water partition coefficients. 2. Hexanol had biphasic effect on erythritol uptake by simple diffusion by red cells. At concentrations less than 9 mM, there was an approximately linear increase in erythritol permeability with increasing alcohol concentration. 3. The facilitated transport of uridine was markedly inhibited by hexanol. Hexanol at 6 mM produced a 65% inhibition of uridine (4 mM) uptake. Hexanol decreased both the apparent Km and V values for the equilibrium exchange of uridine. 4. The facilitated transport of galactose was only slightly inhibited by hexanol. 5. Hexanol was without effect on the passive and active fluxes of Na+ and K+ in red cells with altered cation contents. Cells that were slightly depleted of K+ and cells that were highly K+ -depleted were both insensitive to hexanol.