Membrane lipids as intracellular binders of chlorpromazine and related drugs

The atypical neuroleptics iloperidone and lurasidone inhibit human cytochrome P450 enzymes in vitro. Evaluation of potential metabolic interactions

Background

The present study aimed at examining the inhibitory effect of two atypical neuroleptics iloperidone and lurasidone on the main human cytochrome P450 (CYP) enzymes in pooled human liver microsomes and cDNA-expressed CYP enzymes (supersomes).

Methods

The activity of these enzymes was determined by the following CYP-specific reactions: caffeine 3-N-demethylation/CYP1A2, diclofenac 4′-hydroxylation/CYP2C9, perazine N-demethylation/CYP2C19, bufuralol 1′-hydroxylation/CYP2D6 and testosterone 6β-hydroxylation/CYP3A4, respectively, using HPLC.

Results

Iloperidone inhibited the activity of CYP3A4 via a noncompetitive mechanism (K_i = 0.38 and 0.3 µM in liver microsomes and supersomes, respectively) and CYP2D6 via a competitive mechanism (K_i = 2.9 and 10 µM in microsomes and supersomes). Moreover, iloperidone attenuated the activity of CYP1A2 (K_i = 45 and 31 µM in microsomes and supersomes) and CYP2C19 via a mixed mechanism (K_i = 6.5 and 32 µM in microsomes and supersomes) but did not affect CYP2C9. Lurasidone moderately inhibited CYP1A2 (K_i = 12.6 and 15.5 µM in microsomes and supersomes), CYP2C9 (K_i = 18 and 3.5 µM in microsomes and supersomes) and CYP2C19 via a mixed mechanism (K_i = 18 and 18.4 µM in microsomes and supersomes), and CYP3A4 via a competitive mechanism (K_i = 29.4 and 9.1 µM in microsomes and supersomes). Moreover, lurasidone competitively, though weakly diminished the CYP2D6 activity (K_i = 37.5 and 85 µM in microsomes and supersomes).

Conclusion

The examined neuroleptics showed inhibitory effects on different CYP enzymes. The obtained results indicate that metabolic/pharmacokinetic interactions with iloperidone (involving mainly CYP3A4 and CYP2D6) and possibly with lurasidone (involving CYP1A2, CYP2C9 or CYP2C19) may occur during combined therapy.

In vitro inhibition of human cytochrome P450 enzymes by the novel atypical antipsychotic drug asenapine: a prediction of possible drug-drug interactions

BACKGROUND

Inhibition of cytochrome P450 (CYP) enzymes is the most common cause of harmful drug-drug interactions. The present study aimed at examining the inhibitory effect of the novel antipsychotic drug asenapine on the main CYP enzymes in human liver.

METHODS

The experiments were performed in vitro using pooled human liver microsomes and the human cDNA-expressed CYP enzymes: CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 (Supersomes). Activities of CYP enzymes were determined using the CYP-specific reactions: caffeine 3-N-demethylation (CYP1A2), diclofenac 4'-hydroxylation (CYP2C9), perazine N-demethylation (CYP2C19), bufuralol 1'-hydroxylation (CYP2D6), and testosterone 6β-hydroxylation (CYP3A4). The rates of the CYP-specific reactions were assessed in the absence and presence of asenapine using HPLC.

RESULTS

The obtained results showed that both in human liver microsomes and Supersomes asenapine potently and to a similar degree inhibited the activity of CYP1A2 via a mixed mechanism (K_i = 3.2 μM in liver microsomes and Supersomes) and CYP2D6 via a competitive mechanism (K_i = 1.75 and 1.89 μM in microsomes and Supersomes, respectively). Moreover, asenapine attenuated the CYP3A4 activity via a non-competitive mechanism (K_i = 31.3 and 27.3 μM in microsomes and Supersomes, respectively). In contrast, asenapine did not affect the activity of CYP2C9 or CYP2C19.

CONCLUSION

The potent inhibition of CYP1A2 and CYP2D6 by asenapine, demonstrated in vitro, will most probably be observed also in vivo, since the calculated K_i values are close to the presumed concentration range for asenapine in the liver in vivo. Therefore, pharmacokinetic interactions involving asenapine and CYP2D6 or CYP1A2 substrates are likely to occur during their co-administration to patients.

Thioridazine as Chemotherapy for Mycobacterium avium Complex Diseases

Mycobacterium avium-intracellulare complex (MAC) causes an intractable intracellular infection that presents as chronic pulmonary disease. Currently, therapy consists of ethambutol and macrolides and takes several years to complete. The neuroleptic phenothiazine thioridazine kills mycobacteria by inhibiting the electron transport chain. In several experiments with bacterial populations of up to 10¹² CFU/ml, we failed to isolate any bacteria resistant to 3 times the MIC of thioridazine, suggesting the absence of resistant mutants at bacterial burdens severalfold higher than those encountered in patients. In the hollow-fiber model of intracellular MAC (HFS-MAC), thioridazine achieved an extracellular half-life of 16.8 h and an intracellular half-life of 19.7 h. Thioridazine concentrations were >28,000-fold higher inside infected macrophages than in the HFS-MAC central compartment (equivalent to plasma). Thioridazine maximal kill was 5.20 ± 0.75 log₁₀ CFU/ml on day 7 (r² = 0.96) and 7.19 ± 0.31 log₁₀ CFU/ml on day 14 (r² = 0.99), the highest seen with any drug in the system. Dose fractionation studies revealed that thioridazine efficacy and acquired drug resistance were driven by the peak concentation-to-MIC ratio, with a 50% effective concentration (EC₅₀) of 2.78 ± 0.44 for microbial killing. Acquired drug resistance was encountered by day 21 with suboptimal doses, demonstrating that fluctuating drug concentrations drive evolution faster than static concentrations in mutation frequency studies. However, the thioridazine EC₅₀ changed 16.14-fold when the concentration of fetal bovine serum was changed from 0% to 50%, suggesting that intracellular potency could be heavily curtailed by protein binding. Efficacy in patients will depend on the balance between trapping of the drug in the pulmonary system and the massive intracellular concentrations versus very high protein binding of thioridazine.

Molar ratios of therapeutic water-soluble phenothiazine·water-insoluble phospholipid adducts reveal a Fibonacci correlation and a putative link for structure–activity relationships

The fact that non-antibiotics can sensitise microorganisms for antibiotic treatment suggests that these molecules have valuable potential to treat multiple drug resistance.

Lack of appreciable species differences in nonspecific microsomal binding

Species differences in microsomal binding were evaluated for 43 drug molecules in human, monkey, dog and rat liver microsomes, using a fixed concentration of microsomal protein. The dataset included 32 named drugs and 11 proprietary compounds encompassing a broad spectrum of physicochemical properties (11 acids, 24 bases, 8 neutral, c log D -1 to 7, MW 200 to 700 and free fraction <0.001 to 1). Free fractions (f(u,mic)) in monkey, dog, rat and human microsomes were highly correlated, with linear regression correlation coefficients greater than 0.97. The average fold-difference in f(u,mic) between monkey, dog, or rat, and human was 1.6-, 1.3-, and 1.5-fold, respectively. Species differences in f(u,mic) were also assessed for a range of microsomal protein concentrations (0.2-2 mg/mL) for midazolam, clomipramine, astemizole, and tamoxifen, drugs with low to high microsomal binding. The mean fold species-difference in f(u,mic) for midazolam, clomipramine, astemizole, and tamoxifen was 1.1-, 1.2-, 1.3-, and 2.0-fold, respectively, and was independent of normalized microsomal protein concentration. For a fixed concentration of microsomal protein, greater than 76% and 90% of drugs examined in this study had preclinical species f(u,mic) within 1.5- and 2-fold, respectively, of experimentally measured human values.

Saturable uptake of lipophilic amine drugs into isolated hepatocytes: mechanisms and consequences for quantitative clearance prediction

The hepatic uptake of quinine, fluvoxamine, and fluoxetine (0.1-10 microM) was investigated with freshly isolated rat hepatocytes. The cell-to-medium concentration ratios (K(p)) were concentration-dependent: the mean maximum K(p) values (at 0.1 microM) were 150 (quinine), 500 (fluvoxamine), and 2000 (fluoxetine). There was also a large capacity site that was not saturable over the concentration range used (possibly partition into the phospholipid component of membranes); representing this site, the mean minimum K(p) values (at 10 microM) were 30 (quinine), 200 (fluvoxamine), and 500 (fluoxetine). To eliminate concomitant metabolism, cells were pretreated with the irreversible P450 inhibitor, aminobenzotriazole. The saturable uptake was substantially eliminated after exposure to carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (ATP inhibitor). The difference between the maximum and minimum K(p) for these three amine drugs, as well as for dextromethorphan, propranolol, and imipramine, was within a limited range of 3-fold, indicating a common magnitude of saturable uptake. Basic, permeable drugs are expected to be sequestered into lysosomes, which actively maintain their low internal pH (approximately 5) using ATP, and this process is predictable from the combined effects of pH-driven ion accumulation and unsaturable binding representing partition into membranes. The resultant predicted maximum K(p) correlated strongly with the observed maximum K(p). Thus, at low substrate concentrations, the fraction of drug unbound in the hepatocyte incubation (critical for assessing drug clearance and drug-drug interaction potential) may be dependent upon saturable as well as unsaturable binding, and for lipophilic, basic drugs, this can be readily estimated assuming a common degree of uptake into lysosomes.

Prediction of total propofol clearance based on enzyme activities in microsomes from human kidney and liver

OBJECTIVE

Propofol is commonly used for anesthesia and sedation in intensive care units. Approximately 53% of injected propofol is excreted in the urine as the glucuronide and 38% as hydroxylated metabolites. Liver, kidneys and intestine are suspected as clearance tissues. We investigated the contribution of the liver and kidneys to propofol metabolism in humans using an in vitro-in vivo scale up approach.

METHODS

Renal tissue was obtained from five patients who received nephrectomies. Each renal hydroxylation and glucuronidation enzymatic activities in microsomal fractions from patients were performed discretely and their estimation based on the decrease of propofol concentration. Hepatic hydroxylation and glucuronidation activities were also performed separately using human liver microsomes. This estimation is based on the decrease of propofol concentration, assuming that the contribution of hydroxylation activity without NADPH-generating system and glucuronidation activity without UDPGA in each microsomal fraction are negligible. Both renal and hepatic clearances were estimated assuming a well-stirred model.

RESULTS

Enzymatic activity of propofol oxidation in renal microsomes was negligible. Although glucuronidation activity in microsomes from kidneys was comparable to that from liver, the hepatic intrinsic clearance predicted from in vitro study was higher than that in kidneys due to the larger tissue volume and higher protein concentration. However, glucuronidation clearance in kidney is relatively similar to that in liver because of blood flow limitation of clearance in both tissues.

CONCLUSION

The high degree of hydroxylation activity in liver microsomes is consistent with the blood flow-limited hepatic clearance of propofol. Although the activity of propofol glucuronidation in liver is higher, glucuronidation in kidney may be a substantial contributor.

Antidepressants and antipsychotic drugs colocalize with 5-HT3 receptors in raft-like domains

Despite different chemical structure and pharmacodynamic signaling pathways, a variety of antidepressants and antipsychotics inhibit ion fluxes through 5-HT3 receptors in a noncompetitive manner with the exception of the known competitive antagonists mirtazapine and clozapine. To further investigate the mechanisms underlying the noncompetitive inhibition of the serotonin-evoked cation current, we quantified the concentrations of different types of antidepressants and antipsychotics in fractions of sucrose flotation gradients isolated from HEK293 (human embryonic kidney 293) cells stably transfected with the 5-HT3A receptor and of N1E-115 neuroblastoma cells in relation to the localization of the 5-HT3 receptor protein within the cell membrane. Western blots revealed a localization of the 5-HT3 receptor protein exclusively in the low buoyant density (LBD) fractions compatible with a localization within raft-like domains. Also, the antidepressants desipramine, fluoxetine, and reboxetine and the antipsychotics fluphenazine, haloperidol, and clozapine were markedly enriched in LBD fractions, whereas no accumulation occurs for mirtazapine, carbamazepine, moclobemide, and risperidone. The concentrations of psychopharmacological drugs within LBD fractions was strongly associated with their inhibitory potency against serotonin-induced cation currents. The noncompetitive antagonism of antidepressants at the 5-HT3 receptor was not conferred by an enhancement of receptor internalization as shown by immunofluorescence studies, assessment of receptor density in clathrin-coated vesicles, and electrophysiological recordings after coexpression of a dominant-negative mutant of dynamin I, which inhibits receptor internalization. In conclusion, enrichment of antidepressants and antipsychotics in raft-like domains within the cell membrane appears to be crucial for their antagonistic effects at ligand-gated ion channels such as 5-HT3 receptors.

Ion-Trapping, Microsomal Binding, and Unbound Drug Distribution in the Hepatic Retention of Basic Drugs

This study investigated the relative contribution of ion-trapping, microsomal binding, and distribution of unbound drug as determinants in the hepatic retention of basic drugs in the isolated perfused rat liver. The ionophore monensin was used to abolish the vesicular proton gradient and thus allow an estimation of ion-trapping by acidic hepatic vesicles of cationic drugs. In vitro microsomal studies were used to independently estimate microsomal binding and metabolism. Hepatic vesicular ion-trapping, intrinsic elimination clearance, permeability-surface area product, and intracellular binding were derived using a physiologically based pharmacokinetic model. Modeling showed that the ion-trapping was significantly lower after monensin treatment for atenolol and propranolol, but not for antipyrine. However, no changes induced by monensin treatment were observed in intrinsic clearance, permeability, or binding for the three model drugs. Monensin did not affect binding or metabolic activity in vitro for the drugs. The observed ion-trapping was similar to theoretical values estimated using the pHs and fractional volumes of the acidic vesicles and the pKa values of drugs. Lipophilicity and pKa determined hepatic drug retention: a drug with low pKa and low lipophilicity (e.g., antipyrine) distributes as unbound drug, a drug with high pKa and low lipophilicity (e.g., atenolol) by ion-trapping, and a drug with a high pKa and high lipophilicity (e.g., propranolol) is retained by ion-trapping and intracellular binding. In conclusion, monensin inhibits the ion-trapping of high pKa basic drugs, leading to a reduction in hepatic retention but with no effect on hepatic drug extraction.

Impact of nonspecific binding to microsomes and phospholipid on the inhibition of cytochrome P4502D6: implications for relating in vitro inhibition data to in vivo drug interactions

The effects of microsomal concentration on the inhibitory potencies of four compounds--fluoxetine, quinidine, imipramine, and ezlopitant--on heterologously expressed recombinant CYP2D6-catalyzed bufuralol 1'-hydroxylase activity were determined. Increasing microsomal concentration from 0.0088 to 2.0 mg/ml, using additional microsomes not containing cytochrome P450, resulted in a marked increase in IC(50) and K(I) values for fluoxetine, ezlopitant, and imipramine, when inhibition constants were calculated using the nominal concentration of inhibitor added to the incubation mixture. The extent of nonspecific binding of these inhibitors to microsomes was determined using equilibrium dialysis. The extent of binding increased with increasing microsomal concentration. Binding was greatest for ezlopitant, followed by fluoxetine, imipramine, and quinidine. Correcting inhibition constants for the extent of nonspecific binding resulted in greater consistency of these values with differing microsomal protein concentrations. This effect was also studied with added phospholipid. Inhibition constants increased with increasing phospholipid, and nonspecific binding was also observed for these four drugs to phospholipid. This suggests that the phospholipid component of microsomes possesses some or all of the responsibility for nonspecific binding, and its effect on inhibitors of drug-metabolizing enzymes. These findings suggest that inhibition constants for drugs as inhibitors of microsomal drug-metabolizing enzymes, such as cytochrome P450, should be corrected for the extent of nonspecific binding to components of the in vitro matrix. The implications of this on the prediction of drug-drug interactions from in vitro data are discussed.

Mechanisms of cellular distribution of psychotropic drugs. Significance for drug action and interactions

Distribution of a drug in the body is dependent on its permeation properties, the blood flow rates in various tissues, and on plasma and tissue uptake. The distribution of drugs in vivo is largely determined by uptake competitions between blood and tissues, as well as competitions among individual tissues. Basic lipophilic drugs are characterized by extensive accumulation in tissues, which leads to a high volume of distribution. Nonspecific binding to cellular membranes and uptake by acidic compartments (mainly lysosomes) are responsible for such a distribution pattern. Lysosomal trapping is an important mechanism of distribution of basic psychotropic drugs; however, the tissue distribution of the aliphatic-type phenothiazine neuroleptic promazine, tricyclic antidepressants (TADs) and selective serotonin reuptake inhibitors (SSRIs) depends more on phospholipid binding than on lysosomal trapping, whereas in the case thioridazine and perazine, lysosomal trapping is as important for the tissue uptake as is phospholipid binding. Neuroleptics and antidepressants mutually inhibit their lysosomal uptake. A decrease in the intralysosomal drug concentrations in vivo leads to a shift of the drug from organs abundant in lysosomes (lungs, liver and kidneys) to those poor in these organella, e.g., the heart, which may be of clinical importance (cardiotoxicity). The brain is not a homogenous organ, i.e., the phospholipid pattern and density of lysosomes vary in its different regions. Therefore, the contribution of the two mechanisms (lysosomal trapping and tissue binding) to total drug uptake is different in areas of the brain. Both lysosomal trapping and binding to cellular elements for psychotropics are higher in the grey matter and neurons than in the white matter and astrocytes, respectively. Lysosomal trapping and distribution interactions of psychotropics take place mainly in neurons. A decrease (via a distributive interaction) in the concentration of psychotropics in lysosomes (depot) may lead to an increase in their level in membranes and tissue fluids (i.e., in concentrations and compartments relevant to their pharmacological action) and, in consequence, to enhancement of the drug binding to neurotransmitter receptors and/or transporters.

Lysosomal trapping as an important mechanism involved in the cellular distribution of perazine and in pharmacokinetic interaction with antidepressants

Perazine, a piperazine-type phenothiazine neuroleptic, is the most frequently chosen drug for combination with antidepressants in the therapy of complex or 'treatment-resistant' psychiatric illnesses. The aim of the present study was to investigate the contribution of lysosomal trapping to the total tissue uptake of perazine, and the pharmacokinetic interaction between the neuroleptic and antidepressants. Experiments were carried out on slices of different rat organs regarded as a system with functional lysosomes. To distinguish between lysosomal trapping and tissue binding, the experiments were performed in the absence or presence of 'lysosomal inhibitors', i.e. the lysosomotropic compound ammonium chloride or [H+] ionophore monensin, which abolish the pH-gradient of lysosomes. Under steady-state conditions, the highest tissue uptake of perazine was observed for the adipose tissue, which descended in the following order: the adipose tissue>lungs>liver>heart=brain>kidneys>muscles. The contribution of lysosomal trapping to the total tissue uptake amounted to about 40% in the liver, brain and muscles, to 30% in the kidneys, and to 25% in the heart and lungs. In the adipose tissue, no lysosomotropism of perazine was observed. Of the psychotropics studied, perazine was the only drug showing such a high degree of lysosomal trapping in muscles and distinct lysosomotropic properties in the heart. Perazine and the antidepressants used, both tricyclic (imipramine, amitriptyline) and selective serotonin reuptake inhibitors (fluoxetine, sertraline), mutually decreased their tissue uptake. The potency of imipramine to decrease perazine uptake was similar to that of the 'lysosomal inhibitors'. Other antidepressants seemed to exert a somewhat weaker effect. The above interactions between perazine and antidepressants were not observed in the presence of ammonium chloride, which indicates that they proceeded at the level of lysosomal trapping. The adipose tissue in which the drug uptake was not affected by the 'lysosomal inhibitors' was not the site of such an interaction. Ammonium chloride did not affect the drug metabolism in liver slices; other tissues displayed only a negligible biotransformation of the psychotropics studied. A parallel metabolic interaction between perazine and tricyclic antidepressants took part in liver slices (i.e. perazine and antidepressants mutually inhibited their metabolic pathways), but the influence of such an interaction on the lysosomal uptake of the parent compounds in liver slices did not seem to be great. A substantial decrease in concentrations of the drugs in lysosomes (depot form) observed in vitro may lead to an increase in the concentration in vivo of the neuroleptic and antidepressants at the site of action, which, in turn, may increase the risk of cardiotoxic and anticholinergic side-effects of tricyclic antidepressants and sedative and extrapyramidal effects of the neuroleptic.

The role of lysosomes in the cellular distribution of thioridazine and potential drug interactions

The purpose of the present study was to investigate the contribution of lysosomal trapping to the total tissue uptake of thioridazine and to potential drug distribution interactions between thioridazine and tricyclic antidepressants (imipramine, amitriptyline) or selective serotonin reuptake inhibitors (SSRIs; fluoxetine, sertraline). The experiment was carried out on slices of various rat tissues as a system with intact lysosomes. Thioridazine and antidepressants (5 microM) were incubated separately or jointly with the tissue slices in the absence or presence of "lysosomal inhibitors," i.e., ammonium chloride or monensin. The results show that the contribution of lysosomal trapping to the total tissue uptake of thioridazine is as important as phospholipid binding. A high degree of dependence of thioridazine tissue uptake on the lysosomal trapping is the cause of substantial distributive interactions between thioridazine and the investigated antidepressants at the level of cellular distribution. Thioridazine and the antidepressants, both tricyclic and SSRIs, mutually decreased their tissue uptake. The potency of antidepressants to decrease thioridazine uptake was similar to that of lysosomal inhibitors. In general, the observed interactions between thioridazine and antidepressants occurred only in those tissues in which thioridazine showed lysosomotropism (the lungs, liver, kidneys, brain, and muscles) but were not observed in the presence of ammonium chloride. The above finding provides evidence that the interactions proceeded at the level of lysosomal trapping. In the adipose tissue and heart no lysosomal trapping of thioridazine was detected and those tissues were not the site of such an interaction. Since the organs and tissues involved in the distributive interactions constitute a major part of the organism and take up most of the total drug in the body, the interactions occurring in them may cause a substantial shift of the drugs to organs and tissues poor in lysosomes, e.g. the heart and muscles. An in vivo study into the thioridazine-imipramine interaction showed that joint administration of the drugs under study (10 mg/kg ip) increased drug concentration ratios of lysosome-poor tissue/plasma and lysosome-poor/lysosome-rich tissue. Considering serious side effects of thioridazine and tricyclic antidepressants (cardiotoxicity, anticholinergic activity), the thioridazine-antidepressant combinations studied should be approached with respect to the appropriate dose adjustment.

Contribution of lysosomal trapping to the total tissue uptake of psychotropic drugs

The present study was aimed at assessing individual contributions of the phospholipid binding and lysosomal trapping to the total tissue uptake of psychotropic drugs with different chemical structures, such as promazine, imipramine, amitriptyline, fluoxetine, sertraline (basic lipophilic drugs) and carbamazepine (lipophilic, but not basic). We also tried to find out whether lysosomal trapping may be involved in the pharmacokinetic interactions in clinical combinations of psychotropics. Uptake experiments were carried out on slices of various rat tissues as a system with intact lysosomes. Initial concentration of each drug was 5 microM. The results were compared with those obtained in the presence of the "lysosomal inhibitors", ammonium chloride or monensin. The basic lipophilic psychotropics showed high uptake in tissues known for the abundance of lysosomes, mainly the lungs. The highest drug accumulation was found for promazine and amitriptyline. "Lysosomal inhibitors" significantly decreased the uptake of the basic lipophilic drugs, particularly in the lungs and liver. The most potent effect was observed for amitriptyline, imipramine and promazine. The brain showed moderate accumulation of basic lipophilic psychotropics and the effect of the "lysosomal inhibitors" was significant only in the case of amitriptyline, imipramine and sertraline. The only exception to the above regularity were imipramine and sertraline which were taken up more extensively by the adipose tissue than by lysosome-rich tissues such as the lungs or liver. Carbamazepine did not show lysosomotropism. Amitriptyline and promazine mutually decreased their uptake by lung slices when the drugs were incubated jointly. In the presence of ammonium chloride the interaction did not occur. In conclusion, the obtained results show that (1) the lysosomal trapping is an important factor determining the distribution of the basic lipophilic psychotropics; however (2) their tissue uptake depends more on the phospholipid binding than on the lysosomal trapping; (3) the lysosomal trapping may be involved in the pharmacokinetic interactions between psychotropics.

Axial tissue diffusion can account for the disparity between current models of hepatic elimination for lipophilic drugs

An assumption of previous models of hepatic elimination is that there is negligible axial diffusion in the liver. We show, by construction of a stochastic model and analysis of published data, that compounds which are readily diffusible and partitioned into hepatocytes may undergo axial tissue diffusion. The compounds most likely to be affected by axial tissue diffusion are the lipophilic drugs for which the cell membranes provide little resistance and which are highly extracted, thereby creating steep concentration gradients along the sinusoid at steady state. This phenomenon greatly modifies the availability of the compound under conditions of altered hepatic blood flow and protein binding. For moderately diffusible compounds, these relationships are similar to those predicted by the simplistic venous-equilibrium model. Hence, the paradoxical ability of the venous-equilibrium model to describe the steady-state kinetics of lipophilic drugs such as lidocaine, meperidine, and propranolol may be finally resolved. The effects of axial tissue diffusion and vascular dispersion on hepatic availability of drugs are compared. Vascular dispersion is of major importance to the availability of poorly diffusible compounds, whereas axial tissue diffusion becomes increasingly dominant for highly diffusive and partitioned substances.

The potential role of lysosomes in tissue distribution of weak bases

The potential importance of lysosomes as a site of accumulation of weak bases in tissues is discussed. A simple mathematical treatment predicts the quantitative significance of lysosomal trapping for monoacidic and diacidic weak bases. The features which are characteristics of lysosomal trapping are discussed, particularly in comparison with active transport and intracellular binding mechanisms. These features include: linear accumulation at low concentrations; nonlinearity at higher concentrations; dependence on structural integrity of tissue; energy dependence and competition with other weak bases. Subcellular distribution studies have previously shown that weak bases accumulate extensively in membranes; however, the dependence of accumulation on the structural integrity of tissue suggests that this is not the only significant mechanism of accumulation. The results of a range of studies of tissue distribution of weak bases are discussed to illustrate that these findings are consistent with accumulation in lung and liver being attributable to a combination of lysosomal trapping and accumulation in membranes whereas, in muscle, accumulation in membranes is the predominant mechanism of accumulation. The possible pharmacokinetic significance of lysosomal trapping of weak bases is also discussed.

Binding of organic cations to brush border membrane from rat small intestine

The binding of six organic cations (chlorpromazine, promethazine, imipramine, diphenhydramine, methochlorpromazine and propantheline) to the brush border membrane isolated from rat small intestine has been investigated. The cations were bound to the membrane to varying extents, the order of binding being chlorpromazine greater than promethazine greater than methochlorpromazine greater than imipramine greater than propantheline greater than diphenhydramine. There was no relation between binding and the chloroform-water partition coefficient. Chlorpromazine binding was significantly decreased in the presence of imipramine, methochlorpromazine and propantheline. Anionic compounds (indomethacin and xanthene-9-carboxylic acid) did not affect chlorpromazine binding. High and low affinity binding of the cations to the intestinal brush border membrane was demonstrated with Scatchard plots and Hill plots. Imipramine and methochlorpromazine inhibited chlorpromazine binding at both binding sites. From the results, it was suggested that the organic cations tested were specifically bound to common binding sites on the brush border membrane.

Biphasic effects of chlorpromazine on cell viability in a neuroblastoma cell line

Although chlorpromazine was shown to greatly inhibit a Ca2+-mediated cell death at favorable concentrations (10(-6)-10(-5) M), it caused a drastic decrease in cell viability at higher concentrations (10(-4)-10(-3) M) in a human neuroblastoma cell line. The toxic effect of chlorpromazine also occurred in Ca2+-free medium and was not parallel to the amount of thiobarbituric acid-reactive substances produced. These results indicate that chlorpromazine has biphasic effects on cell viability according to the concentrations added, i.e. a protective effect against cell damage caused by Ca2+, and a direct toxic effect independent of extracellular Ca2+ or of lipid peroxidation.

Binding of propranolol and gentamicin to small unilamellar phospholipid vesicles. Contribution of ionic and hydrophobic forces

Binding of propranolol and gentamicin to small unilamellar phospholipid vesicles having different surface charges was studied at pH 4.4 using an ultra-centrifugation method, and the results were analyzed by an equation describing the Langmuir adsorption isotherms. Gentamicin, a polycationic drug, bound to negatively-charged small unilamellar vesicles composed of 60% phosphatidylcholine and 40% of either phosphatidylinositol, phosphatidylglycerol or phosphatidylserine in a manner consistent with a single class of binding sites but did not bind at all to small unilamellar vesicles of phosphatidylcholine alone. In contrast, propranolol bound readily to both neutral and negatively-charged liposomes in a manner consistent with two types of binding sites. Based on the binding parameters calculated from replots, it is suggested that the high-affinity site is probably at the surface of the liposome and that ionic forces are primarily responsible for this binding. The low-affinity, high-capacity binding site for propranolol was demonstrated with both neutral and negatively-charged liposomes and appeared to be independent of the surface charge. Gentamicin, which is not hydrophobic, did not bind to the low-affinity site. It is hypothesized that hydrophobic interactions are the driving force for propranolol binding to the low-affinity site which may be the interior of the lipid bilayer.

Uptake in vitro of lipophilic model compounds into adipose tissue preparations and lipids

In vitro uptake of 11 lipophilic model compounds into rat epididymal adipose tissue slices, adipocytes, triglycerides, and lecithin was studied. Relative uptake at equilibrium into adipose tissue slices increased from 6 to 87% in the following sequence: phenazone, morphine less than pentobarbital less than glutethimide, phenylbutazone less than thiopental, methadone less than chlorpromazine, imipramine. In the presence of albumin a similar sequence was obtained at lower uptake levels, with DDE and 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) on top with 95% uptake. However, the time to reach equilibrium was unproportionately greater for DDE and 6-CB (16-40 hr) than for other compounds (1-4 hr). A linear positive correlation was found between relative uptake and partition coefficient (octanol/water). Relative uptake was independent of drug concentration. There were no significant differences between uptake values measured with adipose tissue slices, adipocytes, triolein, and a saturated short-chain triglyceride. In contrast, uptake into lecithin was not correlated with the octanol partition coefficient. Thiopental, imipramine, and 6-CB were taken up into lean tissue slices (liver, lung, skin) in excess of their lipid content, suggesting additional binding sites. Release from preloaded adipose tissue slices followed first order kinetics, was accelerated by albumin, and was much slower for 6-CB and DDE than for thiopental and imipramine. The results indicate that uptake of lipophilic xenobiotics in vitro is a partition process between the aqueous medium and the triglyceride of the adipose tissue preparation. In contrast, the extent of adipose tissue storage of drugs in vivo has recently been shown not to correlate with octanol partition coefficients.

Comparison between in-vivo and in-vitro tissue-to-plasma unbound concentration ratios (Kp,f) of quinidine in rats

The comparison between in-vivo and in-vitro tissue-to-plasma concentration ratio of drug unbound (Kp,f) has been made using quinidine as a model for weak basic drugs. In-vitro Kp,f-values were calculated from the binding data to tissue homogenates determined by equilibrium dialysis. In-vivo Kp,f-values were calculated from the tissue distribution data after intravenous administration of quinidine, by considering the difference in the unbound concentration between plasma and the tissues produced by the pH difference across the cell membrane. It was concluded that the extensive tissue distribution of quinidine observed in-vivo may be explained by tissue binding and the pH-difference across the cell membrane in most tissues.

Tissue distribution of chlorimipramine and its demethylated metabolite after a single dose in the rat

The pattern of distribution of chlorimipramine (CI) and of its demethyl-derivative (DMCI) in different organs (lung, liver, kidney, heart and spleen) 24 h after a single dose of CI was examined and related to the amounts of the most representative classes of lipids present in tissues. The findings here reported show that lung and liver have the highest capacity to accumulate CI while DMCI was preferentially accumulated by lung, spleen and kidney. The capacity of the examined tissues to accumulate CI and DMCI did not relate to their lipid content.

Divergent effects of phenothiazines on Leydig tumor cell steroidogenesis and adenylate cyclase activity

The dose and temporal (1-24 h) effects of two phenothiazines, chlorpromazine and trifluoperazine, on steroidogenesis and adenylate cyclase activity of gonadotropin-responsive Leydig tumor cells (M5480A) in primary culture were examined. At low doses (e.g. 0.1-1 microM) these antipsychotic drugs were slightly inhibitory (trifluoperazine) or without effect (chlorpromazine), while at 25 microM each drug was weakly stimulatory to basal testosterone production. Trifluoperazine was, in general, inhibitory to HCG-stimulated testosterone production, but chlorpromazine exhibited paradoxical effects. At 5 and 10 microM this neuroleptic agent increased HCG-stimulated steroidogenesis, while at 25 microM testosterone production was inhibited. In a particulate fraction prepared from the tumor the activity of adenylate cyclase was stimulated 3.4-fold in the presence of 10 microM 5'-guanylimidodiphosphate and 5-fold in the presence of HCG plus the non-hydrolyzable GTP analogue. Between doses of 1-100 microM neither drug altered the basal activity of adenylate cyclase. Trifluoperazine at doses of 1-100 microM inhibited 5'-guanylimidodiphosphate-stimulated adenylate cyclase activity both with and without added gonadotropin. At doses of 1-10 microM chlorpromazine had no effect on adenylate cyclase activity, but it stimulated activity in the dose range of 20-100 microM. Interestingly, in the presence of 5'-guanylimidodiphosphate this drug did not alter the stimulated enzymic activity achieved with a maximal dose of HCG. Therefore, these phenothiazines exhibit quite divergent dose-dependent effects and their actions must occur at multiple loci. Also, it seems unlikely that the effects of these agents on steroidogenesis and adenylate cyclase activity can be reconciled solely in terms of calmodulin-mediated processes.

African trypanosomes contain calmodulin which is distinct from host calmodulin

Studies were initiated to determine whether African trypanosomes utilize Ca2+ fluxes to coordinate complex morphological and biochemical life cycle changes. We have identified the ubiquitous intracellular Ca2+ receptor, calmodulin, in two developmental stages of Trypanosoma brucei rhodesiense. The transition from rapidly dividing, slender bloodstream trypomastigotes to slow growing procyclics in axenic culture was accompanied by changes in specific calmodulin content (3 micrograms/mg cell protein to 1 microgram/mg cell protein, respectively) and a shift in intracellular calmodulin distribution, Trypanosome calmodulin is physically and functionally distinct from that of host tissues, including bovine brain and rat erythrocytes. It is similar to but distinct from Tetrahymena calmodulin. Comparisons among these proteins isolated from the four sources were made using the following criteria: (1) mobility on sodium dodecyl sulfate discontinuous polyacrylamide gels; (2) Ca2+-induced conformational changes; (3) CNBr-cleavage fragments; (4) activation of bovine brain cyclic nucleotide phosphodiesterase in both a Ca2+-dependent and calmodulin-dependent manner; (5) activation of human erythrocyte (Ca2+ + Mg2+)-ATPase; and (6) inhibition of calmodulin activity by trifluoperazine and penfluridol. Trifluoperazine but not trifluoperazine sulfoxide was cytotoxic to trypanosomes in vitro. Half maximal effect occurred at 15 microM. We conclude that calmodulin is a functional component of Africal trypanosomes and suggest that it plays an important role in mediating the host-parasite relationship.

A technique for the comparison of biological distribution and solvent partition of drugs

A dialysis technique is described which allows the measurement of drug distribution between buffer and solvents as well as between buffer and biological preparations under identical experimental conditions. Partition and distribution coefficients of thiopental, pentobarbital, imipramine, and chlorpromazine were determined using octanol, other solvents, and tissue homogenates.

Interactions between bis(guanylhydrazones) and polyamines in isolated mitochondria

The interactions of naturally occurring polyamines: putrescine, spermidine and spermine, with anticancer bis-guanylhydrazones: methylglyoxal-bis(guanylhydrazone) (MGBG) and 4,4'-diacetyldiphenylurea-bis(guanylhydrazone) (DDUG) were investigated at the level of mitochondrial membrane. The effects of bis-guanylhydrazones on intact rat liver mitochondria were readily prevented or reversed by polyamines and these interactions were also affected by the mitochondrial transmembrane potential. Magnesium cations enhanced the protective action of polyamines. The data indicate that competition exists between the essential anticancer bis(guanylhydrazone) and polyamines for low affinity negatively charged binding sites at the outer surface of inner mitochondrial membrane. The study of drug interactions was extended to the level of isolated tumor mitochondria from rat HTC hepatoma and murine L1210 leukemia cells. A complicated pattern of interactions between the anticancer bis-guanylhydrazones and phenethylbiguanide was obtained.

Effect of chlorpromazine on isolated rat hepatocytes

The effect of chlorpromazine hydrochloride (CPZ) (1-500 microM) on plasma membrane permeability and mitochondrial respiratory function of isolated rat hepatocytes was studied. The endogenous oxygen consumption stimulated by 1 mM succinate was increased significantly by 5 microM CPZ, whereas the ability to exclude trypan blue (TB) was decreased significantly by 100 microM CPZ. The release of a cytosomal enzyme, lactate dehydrogenase (LDH), was increased significantly by 50 microM CPZ, whereas the release of glutamic-opalacetic transaminase (GOT) was increased significantly by 100 microM. The endogenous oxygen consumption was decreased significantly by 150 microM CPZ. The respiration control ratio by 2 microM carbonylcyanide-m-chlorphenyl hydrazon (CCP) showed significant decreases at all concentrations of CPZ studied; and this might be attributable to the suppression by CPZ of the respiratory stimulation induced by CCP. The results indicated that CPZ at a low concentration (5 microM) first produced a significant change in plasma membrane permeability to low molecular substances such as succinate and then at higher concentrations (50-100 microM) produced significant release of the cytosomal and mitochondrial enzymes, LDH and GOT. They also indicated that the concentrations of CPZ which produced significant effects on respiratory function were higher (above 150 microM) than those which produced significant changes in plasma membrane permeability of hepatocytes.

Phenothiazines and related compounds disrupt mitochondrial energy production by a calmodulin-independent reaction

Phenothiazines and related compounds bind to mitochondrial membranes in approximate proportion to their affinities for calmodulin. Penfluridol (16 microM), pimozide (20 microM), or trifluoperazine (66 microM) completely inhibit ADP-stimulated respiration in isolated rat liver mitochondria, but exert no effect on either uncoupler- or Ca2+-stimulated respiration. The inhibition of ADP-stimulated respiration results from inhibition of the oligomycin-sensitive ATPase. Inhibition of the ATPase does not involve interaction of phenothiazine with calmodulin. The addition of calmodulin with or without calcium to mitochondrial inner membrane preparations has no effect on ATPase activity. The addition of EGTA and the ionophore A23187 prior to the addition of phenothiazine does not prevent the phenothiazine-induced inhibiton of the ATPase. Measurements of inner membrane calmodulin content by gel electrophoresis or cyclic nucleotide phosphodiesterase activation are negative. Despite the absence of calmodulin in the inner membrane preparations, 12.5 nmol trifluoperazine bind per 100 microgram of membrane protein with an association constant, K, of 6.5 . 10(4) M-1. We conclude that calmodulin-binding neuroleptic agents, when added to whole cells, have the potential to disrupt mitochondrial energy production by a reaction which apparently does not involve a phenothiazine-calmodulin interaction.

Pharmacokinetics in rats of 2,4,5,2',4',5'-hexachlorobiphenyl, an unmetabolizable lipophilic model compound

1. Pharmacokinetics of the unmetabolizable lipophilic model compound, 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) was studied in rats, using g.l.c. and 14C methods. 2. After single i.v. doses of 0.6 and 3.6 mg/kg, 16% dose was excreted in 40 weeks in the faeces; the value for infinite time was 17% dose. This limited excretion was first-order with a half-life of 100 days for the terminal component. Urinary excretion was nearly complete after 1 week and amounted to 0.8% dose. 3. 6-CB was redistributed from blood to liver, muscle, skin, and adipose tissue. The latter contained a constant level of about 75% dose from 6 to 40 weeks, while the total lean tissue level fell to 6% dose; only 6-CB in the lean tissue compartment was available for excretion. 4. In rats given six oral doses of 0.6 mg/kg at weekly intervals, excretion and distribution patterns were similar to the single-dose situation, and were thus independent of dose, route of administration, and dose regimen. 5. It is concluded that in rats under physiological conditions, about 75% of every dose of 6-CB is irreversibly stored in adipose tissue and that excretion is limited to 18% dose. 6-CB in rats exhibit novel pharmacokinetics of unmetabolizable lipophilic compounds.

Drug distribution as a function of binding competition. Experiments with the distribution dialysis technique

In the distribution dialysis technique each of the two dialysis chambers contains a binding system, and a drug is allowed to distribute between them. This technique was tested by using various intracellular and extracellular binder preparations over wide concentration ranges, and model drugs selected for their known binding properties. The drugs were then tested at therapeutic concentrations in standardized systems of liver homogenate (0.5 g ml-1) and whole blood (0.02 ml ml-1). The resulting intracellular/extracellular concentrations ratios were characteristic for the binding properties of the various drugs. Thus, for imipramine, a drug with strong tissue and weaker plasma binding properties, the concentration ratios were 25 for the system homogenate/buffer, 0.8 for buffer/blood, and 15 for the competitive system homogenate/blood. In experiments with homogenates from various tissues (liver, lung, kidney, intestine, brain) and blood in the standard system, the following approximate ratios were obtained: 1 for antipyrine, 2 for phenylbutazone, 14 for imipramine (but only 8 with muscle, skin and adipose tissue). These results reflect both the individual binding to intracellular and extracellular components and the tissue/blood concentration ratios in vivo. It is suggested that distribution dialysis is an in vitro method for characterizing the distribution of the drugs. It is also concluded that drug distribution is largely determined by a binding competition between tissue and blood sites.

Interaction of chlorpromazine with organic solvents and fatty acids as studied by UV-spectrophotometry

The magnitude of the UV-spectral change of chlorpromazine increases in the presence of increasing concentrations of alcohols or fatty acids and with increasing chain length. A maximum is reached with 14.0- or 16.0-fatty acids. The differential spectrum is still larger with unsaturated fatty acids, a maximum effect being obtained with one cis-double bond. The spectral change is abolished by chaotropic and enhanced by antichaotropic agents.