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

Weak base drug-induced endolysosome iron dyshomeostasis controls the generation of reactive oxygen species, mitochondrial depolarization, and cytotoxicity

Objectives

Approximately 75 % of marketed drugs have the physicochemical property of being weak bases. Weak-base drugs with relatively high pKa values enter acidic organelles including endosomes and lysosomes (endolysosomes), reside in and de-acidify endolysosomes, and induce cytotoxicity. Divalent cations within endolysosomes, including iron, are released upon endolysosome de-acidification. Endolysosomes are "master regulators of iron homeostasis", and neurodegeneration is linked to ferrous iron (Fe2+)-induced reactive oxygen species (ROS) generation via Fenton chemistry. Because endolysosome de-acidification-induced lysosome-stress responses release endolysosome Fe2+, it was crucial to determine the mechanisms by which a functionally and structurally diverse group of weak base drugs including atropine, azithromycin, fluoxetine, metoprolol, and tamoxifen influence endolysosomes and cause cell death.

Methods

Using U87MG astrocytoma and SH-SY5Y neuroblastoma cells, we conducted concentration-response relationships for 5 weak-base drugs to determine EC50 values. From these curves, we chose pharmacologically and therapeutically relevant concentrations to determine if weak-base drugs induced lysosome-stress responses by de-acidifying endolysosomes, releasing endolysosome Fe2+ in sufficient levels to increase cytosolic and mitochondria Fe2+ and ROS levels and cell death.

Results

Atropine (anticholinergic), azithromycin (antibiotic), fluoxetine (antidepressant), metoprolol (beta-adrenergic), and tamoxifen (anti-estrogen) at pharmacologically and therapeutically relevant concentrations (1) de-acidified endolysosomes, (2) decreased Fe2+ levels in endolysosomes, (3) increased Fe2+ and ROS levels in cytosol and mitochondria, (4) induced mitochondrial membrane potential depolarization, and (5) increased cell death; effects prevented by the endocytosed iron-chelator deferoxamine.

Conclusions

Weak-base pharmaceuticals induce lysosome-stress responses that may affect their safety profiles; a better understanding of weak-base drugs on Fe2+ interorganellar signaling may improve pharmacotherapeutics.

Antipsychotics cardiotoxicity: What's known and what's next

Chronic use of antipsychotic medications entails a dilemma between the benefit of alleviating psychotic symptoms and the risk of troubling, sometimes life-shortening adverse effects. Antipsychotic-induced cardiotoxicity is one of the most life-threatening adverse effects that raises widespread concerns. These cardiotoxic effects range from arrhythmia to heart failure in the clinic, with myocarditis/cardiomyopathy, ischemic injuries, and unexplained cardiac lesions as the pathological bases. Multiple mechanisms have been proposed to underlie antipsychotic cardiotoxicity. This review aims to summarize the clinical signs and pathological changes of antipsychotic cardiotoxicity and introduce recent progress in understanding the underlying mechanisms at both the subcellular organelle level and the molecular level. We also provide an up-to-date perspective on future clinical monitoring and therapeutic strategies for antipsychotic cardiotoxicity. We propose that third-generation antipsychotics or drug adjuvant therapy, such as cannabinoid receptor modulators that confer dual benefits — i.e., alleviating cardiotoxicity and improving metabolic disorders — deserve further clinical evaluation and marketing.

The role of endolysosomal trafficking in anticancer drug resistance

Multidrug resistance (MDR) remains a major obstacle towards curative treatment of cancer. Despite considerable progress in delineating the basis of intrinsic and acquired MDR, the underlying molecular mechanisms remain to be elucidated. Emerging evidences suggest that dysregulation in endolysosomal compartments is involved in mediating MDR through multiple mechanisms, such as alterations in endosomes, lysosomes and autophagosomes, that traffic and biodegrade the molecular cargo through macropinocytosis, autophagy and endocytosis. For example, altered lysosomal pH, in combination with transcription factor EB (TFEB)-mediated lysosomal biogenesis, increases the sequestration of hydrophobic anti-cancer drugs that are weak bases, thereby producing an insufficient and off-target accumulation of anti-cancer drugs in MDR cancer cells. Thus, the use of well-tolerated, alkalinizing compounds that selectively block Vacuolar H⁺-ATPase (V-ATPase) may be an important strategy to overcome MDR in cancer cells and increase chemotherapeutic efficacy. Other mechanisms of endolysosomal-mediated drug resistance include increases in the expression of lysosomal proteases and cathepsins that are involved in mediating carcinogenesis and chemoresistance. Therefore, blocking the trafficking and maturation of lysosomal proteases or direct inhibition of cathepsin activity in the cytosol may represent novel therapeutic modalities to overcome MDR. Furthermore, endolysosomal compartments involved in catabolic pathways, such as macropinocytosis and autophagy, are also shown to be involved in the development of MDR. Here, we review the role of endolysosomal trafficking in MDR development and discuss how targeting endolysosomal pathways could emerge as a new therapeutic strategy to overcome chemoresistance in cancer.

Propranolol Sensitizes Vascular Sarcoma Cells to Doxorubicin by Altering Lysosomal Drug Sequestration and Drug Efflux

Angiosarcoma is a rare cancer of blood vessel-forming cells with a high patient mortality and few treatment options. Although chemotherapy often produces initial clinical responses, outcomes remain poor, largely due to the development of drug resistance. We previously identified a subset of doxorubicin-resistant cells in human angiosarcoma and canine hemangiosarcoma cell lines that exhibit high lysosomal accumulation of doxorubicin. Hydrophobic, weak base chemotherapeutics, like doxorubicin, are known to sequester within lysosomes, promoting resistance by limiting drug accessibility to cellular targets. Drug synergy between the beta adrenergic receptor (β-AR) antagonist, propranolol, and multiple chemotherapeutics has been documented in vitro, and clinical data have corroborated the increased therapeutic potential of propranolol with chemotherapy in angiosarcoma patients. Because propranolol is also a weak base and accumulates in lysosomes, we sought to determine whether propranolol enhanced doxorubicin cytotoxicity via antagonism of β-ARs or by preventing the lysosomal accumulation of doxorubicin. β-AR-like immunoreactivities were confirmed in primary tumor tissues and cell lines; receptor function was verified by monitoring downstream signaling pathways of β-ARs in response to receptor agonists and antagonists. Mechanistically, propranolol increased cytoplasmic doxorubicin concentrations in sarcoma cells by decreasing the lysosomal accumulation and cellular efflux of this chemotherapeutic agent. Equivalent concentrations of the receptor-active S-(-) and -inactive R-(+) enantiomers of propranolol produced similar effects, supporting a β-AR-independent mechanism. Long-term exposure of hemangiosarcoma cells to propranolol expanded both lysosomal size and number, yet cells remained sensitive to doxorubicin in the presence of propranolol. In contrast, removal of propranolol increased cellular resistance to doxorubicin, underscoring lysosomal doxorubicin sequestration as a key mechanism of resistance. Our results support the repurposing of the R-(+) enantiomer of propranolol with weak base chemotherapeutics to increase cytotoxicity and reduce the development of drug-resistant cell populations without the cardiovascular and other side effects associated with antagonism of β-ARs.

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 inhibits autophagy and sensitizes glioblastoma cells to temozolomide

Glioblastoma multiforme (GBM) has a poor prognosis with an overall survival of 14-15 months after surgery, radiation and chemotherapy using temozolomide (TMZ). A major problem is that the tumors acquire resistance to therapy. In an effort to improve the therapeutic efficacy of TMZ, we performed a genome-wide RNA interference (RNAi) synthetic lethality screen to establish a functional gene signature for TMZ sensitivity in human GBM cells. We then queried the Connectivity Map database to search for drugs that would induce corresponding changes in gene expression. By this approach we identified several potential pharmacological sensitizers to TMZ, where the most potent drug was the established antipsychotic agent Thioridazine, which significantly improved TMZ sensitivity while not demonstrating any significant toxicity alone. Mechanistically, we show that the specific chemosensitizing effect of Thioridazine is mediated by impairing autophagy, thereby preventing adaptive metabolic alterations associated with TMZ resistance. Moreover, we demonstrate that Thioridazine inhibits late-stage autophagy by impairing fusion between autophagosomes and lysosomes. Finally, Thioridazine in combination with TMZ significantly inhibits brain tumor growth in vivo, demonstrating the potential clinical benefits of compounds targeting the autophagy-lysosome pathway. Our study emphasizes the feasibility of exploiting drug repurposing for the design of novel therapeutic strategies for GBM.

Quantitation of Lysosomal Trapping of Basic Lipophilic Compounds Using In Vitro Assays and In Silico Predictions Based on the Determination of the Full pH Profile of the Endo-/Lysosomal System in Rat Hepatocytes

Lysosomal sequestration may affect the pharmacokinetics, efficacy, and safety of new basic lipophilic drug candidates potentially impacting their intracellular concentrations and tissue distribution. It may also be involved in drug-drug interactions, drug resistance, and phospholipidosis. However, currently there are no assays to evaluate the lysosomotropic behavior of compounds in a setting fully meeting the needs of drug discovery. We have, therefore, integrated a set of methods to reliably rank order, quantify, and calculate the extent of lysosomal sequestration in rat hepatocytes. An indirect fluorescence-based assay monitors the displacement of the fluorescence probe LysoTracker Red by test compounds. Using a lysosomal-specific evaluation algorithm allows one to generate IC₅₀ values at lower than previously reported concentrations. The concentration range directly agrees with the concentration dependency of the lysosomal drug content itself directly quantified by liquid chromatography-tandem mass spectrometry and thus permits a quantitative link between the indirect and the direct trapping assay. Furthermore, we have determined the full pH profile and corresponding volume fractions of the endo-/lysosomal system in plated rat hepatocytes, enabling a more accurate in silico prediction of the extent of lysosomal trapping based only on pK _a values as input, allowing early predictions even prior to chemical synthesis. The concentration dependency-i.e., the saturability of the trapping-can then be determined by the IC₅₀ values generated in vitro. Thereby, a more quantitative assessment of the susceptibility of basic lipophilic compounds for lysosomal trapping is possible.

Systemic thioridazine in combination with dicloxacillin against early aortic graft infections caused by Staphylococcus aureus in a porcine model: In vivo results do not reproduce the in vitro synergistic activity

Introduction

Conservative treatment solutions against aortic prosthetic vascular graft infection (APVGI) for inoperable patients are limited. The combination of antibiotics with antibacterial helper compounds, such as the neuroleptic drug thioridazine (TDZ), should be explored.

Aim

To investigate the efficacy of conservative systemic treatment with dicloxacillin (DCX) in combination with TDZ (DCX+TDZ), compared to DCX alone, against early APVGI caused by methicillin-sensitive Staphylococcus aureus (MSSA) in a porcine model.

Methods

The synergism of DCX+TDZ against MSSA was initially assessed in vitro by viability assay. Thereafter, thirty-two pigs had polyester grafts implanted in the infrarenal aorta, followed by inoculation with 10⁶ CFU of MSSA, and were randomly administered oral systemic treatment with either 1) DCX or 2) DCX+TDZ. Treatment was initiated one week postoperatively and continued for a further 21 days. Weight, temperature, and blood samples were collected at predefined intervals. By termination, bacterial quantities from the graft surface, graft material, and perigraft tissue were obtained.

Results

Despite in vitro synergism, the porcine experiment revealed no statistical differences for bacteriological endpoints between the two treatment groups, and none of the treatments eradicated the APVGI. Accordingly, the mixed model analyses of weight, temperature, and blood samples revealed no statistical differences.

Conclusion

Conservative systemic treatment with DCX+TDZ did not reproduce in vitro results against APVGI caused by MSSA in this porcine model. However, unexpected severe adverse effects related to the planned dose of TDZ required a considerable reduction to the administered dose of TDZ, which may have compromised the results.

Thioridazine: A Non-Antibiotic Drug Highly Effective, in Combination with First Line Anti-Tuberculosis Drugs, against Any Form of Antibiotic Resistance of Mycobacterium tuberculosis Due to Its Multi-Mechanisms of Action

This review presents the evidence that supports the use of thioridazine (TZ) for the therapy of a pulmonary tuberculosis infection regardless of its antibiotic resistance status. The evidence consists of in vitro and ex vivo assays that demonstrate the activity of TZ against all encountered Mycobacterium tuberculosis (Mtb) regardless of its antibiotic resistance phenotype, as well as in vivo as a therapy for mice infected with multi-drug resistant strains of Mtb, or for human subjects infected with extensively drug resistant (XDR) Mtb. The mechanisms of action by which TZ brings about successful therapeutic outcomes are presented in detail.

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.

Ion Channel Blockers as Antimicrobial Agents, Efflux Inhibitors, and Enhancers of Macrophage Killing Activity against Drug Resistant Mycobacterium tuberculosis

Given the ability of M. tuberculosis to survive as an intracellular pathogen and its propensity to develop resistance to the existing antituberculosis drugs, its treatment requires new approaches. Here the antimycobacterial properties of verapamil, thioridazine, chlorpromazine, flupenthixol and haloperidol were investigated against a panel of drug resistant M. tuberculosis strains, both in vitro and on human-infected macrophages. These compounds are efflux inhibitors that share among them the characteristic of being ion channel blockers. In vitro, all compounds exhibited synergistic inhibitory activities when combined with isoniazid and rifampicin, and were able to inhibit active efflux, demonstrating their role as efflux inhibitors. Gene expression analysis showed that M. tuberculosis efflux genes were overexpressed in response to antibiotic exposure, in vitro and within macrophages, irrespective of their resistance pattern. These compounds displayed a rapid and high killing activity against M. tuberculosis, associated with a decrease in intracellular ATP levels demonstrating that the bactericidal action of the ion channel blockers against M. tuberculosis clinical strains is associated with their interference with energy metabolism. The compounds led to a decrease in the intracellular mycobacterial load by increasing phagosome acidification and activating lysosomal hydrolases. The results presented in this study enable us to propose the following mechanism of action for these compounds: a) in the bacteria, the compounds generate a cascade of events involving the inhibition of the respiratory chain complexes and energy production for efflux activity. Indirectly, this reduce the resistance level to antituberculosis drugs potentiating their activity; b) on the host cell, the treatment with the ion channel blockers increases phagosome acidification and induces the expression of phagosomal hydrolases, leading to bacterial growth restriction irrespective of their resistance pattern. This work highlights the potential value ion channel blockers as adjuvants of tuberculosis chemotherapy, in particular for the development of new therapeutic strategies, with strong potential for treatment shortening against drug susceptible and resistant forms of tuberculosis. Medicinal chemistry studies are now needed to improve the properties of these compounds, increasing their M. tuberculosis efflux-inhibition and killing-enhancement activity and reduce their toxicity for humans, therefore optimizing their potential for clinical usage.

Assessments of Thioridazine as a Helper Compound to Dicloxacillin against Methicillin-Resistant Staphylococcus aureus: In Vivo Trials in a Mouse Peritonitis Model

Introduction

The rise in antimicrobial resistance is a major global concern and requires new treatment strategies. The use of helper compounds, such as thioridazine (TDZ), an antipsychotic drug, in combination with traditional antibiotics must be investigated.

Objectives

The aim of this study was to investigate the efficacy of TDZ as a helper compound for dicloxacillin (DCX) against methicillin-resistant Staphylococcus aureus (MRSA) in vivo, and compare the combination treatment of DCX+TDZ with vancomycin (VAN).

Methods

Mice were inoculated with an intraperitoneal (IP) injection of MRSA (10⁸ CFU) and treated in a 12-hour cycle for 48 hours. By termination, bacterial quantities in a peritoneal flush, spleen and kidneys were obtained. In the main trial the drugs were administered subcutaneously in five treatment groups: 1) DCX, 2) TDZ, 3) DCX+TDZ, 4) VAN, 5) SALINE. Additional smaller studies with IP administration and higher subcutaneous dosages (×1.5 and ×4) of the drugs were subsequently performed.

Results

In the main trial no significant differences were found between DCX+TDZ and DCX or TDZ alone (p≥0.121–0.999). VAN performed significantly better than DCX+TDZ on all bacteriological endpoints (p<0.001). Higher subcutaneous dosages of DCX and TDZ improved the antibacterial efficacy, but the combination treatment was still not significantly better than monotherapy. IP drug administration of DCX+TDZ revealed a significantly better antibacterial effect than DCX or TDZ alone (p<0.001) but not significantly different from VAN (p>0.999).

Conclusion

In conclusion, TDZ did not prove to be a viable helper compound for dicloxacillin against MRSA in subcutaneous systemic treatment. However, IP-administration of DCX+TDZ, directly at the infection site resulted in a synergetic effect, with efficacy comparable to that of VAN.

Time-dependent effects of hydrophobic amine-containing drugs on lysosome structure and biogenesis in cultured human fibroblasts

Many weakly basic amine-containing drugs are known to be extensively sequestered in acidic lysosomes by an ion trapping-type mechanism. The entrapment of drugs in lysosomes has been shown to influence drug activity, cancer cell selectivity, and pharmacokinetics and can cause the hyperaccumulation of various lipids associated with lysosomes. In this work, we have investigated the prolonged time-dependent effects of drugs on lysosomal properties. We have evaluated two amine-containing drugs with intermediate (propranolol) and high (halofantrine) relative degrees of lipophilicity. Interestingly, the cellular accumulation kinetics of these drugs exhibited a biphasic characteristic at therapeutically relevant exposure levels with an initial apparent steady-state occurring at 2 days followed by a second stage of enhanced accumulation. We provide evidence that this secondary drug accumulation coincides with the nuclear localization of transcription factor EB, a master regulator of lysosome biogenesis, and the appearance of an increased number of smaller and lipid-laden lysosomes. Collectively, these results show that hydrophobic lysosomotropic drugs can induce their own cellular accumulation in a time-dependent fashion and that this is associated with an expanded lysosomal volume. These results have important therapeutic implications and may help to explain sources of variability in drug pharmacokinetic distribution and elimination properties observed in vivo.

Di-22:6-bis(monoacylglycerol)phosphate: A clinical biomarker of drug-induced phospholipidosis for drug development and safety assessment

The inability to routinely monitor drug-induced phospholipidosis (DIPL) presents a challenge in pharmaceutical drug development and in the clinic. Several nonclinical studies have shown di-docosahexaenoyl (22:6) bis(monoacylglycerol) phosphate (di-22:6-BMP) to be a reliable biomarker of tissue DIPL that can be monitored in the plasma/serum and urine. The aim of this study was to show the relevance of di-22:6-BMP as a DIPL biomarker for drug development and safety assessment in humans. DIPL shares many similarities with the inherited lysosomal storage disorder Niemann-Pick type C (NPC) disease. DIPL and NPC result in similar changes in lysosomal function and cholesterol status that lead to the accumulation of multi-lamellar bodies (myeloid bodies) in cells and tissues. To validate di-22:6-BMP as a biomarker of DIPL for clinical studies, NPC patients and healthy donors were classified by receiver operator curve analysis based on urinary di-22:6-BMP concentrations. By showing 96.7-specificity and 100-sensitivity to identify NPC disease, di-22:6-BMP can be used to assess DIPL in human studies. The mean concentration of di-22:6-BMP in the urine of NPC patients was 51.4-fold (p ≤ 0.05) above the healthy baseline range. Additionally, baseline levels of di-22:6-BMP were assessed in healthy non-medicated laboratory animals (rats, mice, dogs, and monkeys) and human subjects to define normal reference ranges for nonclinical/clinical studies. The baseline ranges of di-22:6-BMP in the plasma, serum, and urine of humans and laboratory animals were species dependent. The results of this study support the role of di-22:6-BMP as a biomarker of DIPL for pharmaceutical drug development and health care settings.

Efflux pumps of Gram-negative bacteria: what they do, how they do it, with what and how to deal with them

This review discusses the relationship of the efflux pump (EP) system of Gram-negative bacteria to other antibiotic resistance mechanisms of the bacterium such as quorum sensing, biofilms, two component regulons, etc. The genetic responses of a Gram-negative to an antibiotic that render it immune to an antibiotic are also discussed. Lastly, the methods that have been developed for the identification of bacteria that over-express their EP system are presented in detail. Phenothiazines are well-known antipsychotic drugs with reported activity against bacterial EPs and other ancillary antibiotic mechanisms of the organism. Therefore these compounds will also be discussed.

Thioridazine pharmacokinetic-pharmacodynamic parameters "Wobble" during treatment of tuberculosis: a theoretical basis for shorter-duration curative monotherapy with congeners

Phenothiazines are being repurposed for treatment of tuberculosis. We examined time-kill curves of thioridazine and first-line drugs against log-growth-phase and semidormant bacilli under acidic conditions and nonreplicating persistent Mycobacterium tuberculosis. While both the potency and the efficacy of first-line drugs declined dramatically as M. tuberculosis replication rates decreased, those of thioridazine improved. The mutation prevalence to 3 times the thioridazine MIC was <1 × 10(-11), better than for ≥2 first-line drugs combined. Hollow fiber system studies revealed that the relationship between sterilizing effect and pharmacodynamic indices (PDI) was characterized by an r(2) of 0.88 for peak/MIC, an r(2) of 0.47 for the area under the concentration-time curve (AUC) to MIC, and an r(2) of 0.14 for the cumulative percentage of a 24-h period that the drug concentration exceeds the MIC under steady-state pharmacokinetic conditions (%TMIC) at the end of the first week. However, the PDI linked to effect "wobbled" as the duration of therapy increased, so that by the fourth week the r(2) was 0.88 for AUC/MIC, 0.78 for %TMIC, and 0.72 for peak/MIC. This "wobble" has implications on general pharmacokinetic/pharmacodynamic theory, whereby efficacy is linked to only one of the three PDIs in deterministic models. The potency changed 8.9-fold from the first to the fourth weeks. The non-protein-bound AUC/MIC associated with maximal kill at the end of therapy was 50.53 (protein binding = 99.5%). This thioridazine exposure was calculated to extinguish all three M. tuberculosis metabolic populations in human lungs in only 42.9 days of monotherapy. However, this concentration exceeds the 2- to 8-mg/liter thioridazine concentration in serum known to be lethal to humans. Therefore, the way forward for phenothiazine monotherapy that also reduces therapy duration is via synthesis of less toxic congeners.

Differential cytotoxicity responses by dog and rat hepatocytes to phospholipogenic treatments

Dog and rat hepatocytes were treated with phospholipogenics to identify the more sensitive species and to determine whether lysosomal or mitochondrial changes were the primary cause of cytotoxicity. Endpoints included cell death, lysosome membrane integrity, mitochondrial membrane polarization, and fluorescent phospholipid (NBD-PE). Dog cells exhibited lower survival IC₅₀ values than did rat cells with all phospholipogenic treatments and exhibited a lower capacity to accumulate NBD-PE in 4 of 5 phospholipogenic test conditions. The lysosomal modulator Bafilomycin A1 (Baf) rescued dog cells from cytotoxicity caused by 3 phospholipogenic 5HT1_b antagonists and hydroxychloroquine, but not fluoxetine, and rescued rat cells from hydroxychloroquine and NMTMB, a 5HT1_b antagonist. Following NMTMB treatment, rat mitochondrial membrane hyperpolarization was observed at modestly cytotoxic concentrations and depolarization at the highest concentration. At the highest test concentration, lysosomal loss of acridine orange occurred by 30 min, mitochondrial polarity changes by 1 hr, and NBD-PE accumulation by 2 hr, respectively. Baf shifted mitochondrial polarity from a depolarized state to a hyperpolarized state. These data demonstrate that (a) dog hepatocytes were generally less capable of mounting an adaptive, protective phospholipidotic response than rat hepatocytes, (b) effects on mitochondria and survival were preventable by lysosomal protection, and (c) destabilizing changes in both organelles are involved causally in cytotoxicity.

Lysosomal sequestration (trapping) of lipophilic amine (cationic amphiphilic) drugs in immortalized human hepatocytes (Fa2N-4 cells)

Lipophilic (logP > 1) and amphiphilic drugs (also known as cationic amphiphilic drugs) with ionizable amines (pKa > 6) can accumulate in lysosomes, a process known as lysosomal trapping. This process contributes to presystemic extraction by lysosome-rich organs (such as liver and lung), which, together with the binding of lipophilic amines to phospholipids, contributes to the large volume of distribution characteristic of numerous cardiovascular and central nervous system drugs. Accumulation of lipophilic amines in lysosomes has been implicated as a cause of phospholipidosis. Furthermore, elevated levels of lipophilic amines in lysosomes can lead to high organ-to-blood ratios of drugs that can be mistaken for active drug transport. In the present study, we describe an in vitro fluorescence-based method (using the lysosome-specific probe LysoTracker Red) to identify lysosomotropic agents in immortalized hepatocytes (Fa2N-4 cells). A diverse set of compounds with various physicochemical properties were tested, such as acids, bases, and zwitterions. In addition, the partitioning of the nonlysosomotropic atorvastatin (an anion) and the lysosomotropics propranolol and imipramine (cations) were quantified in Fa2N-4 cells in the presence or absence of various lysosomotropic or nonlysosomotropic agents and inhibitors of lysosomal sequestration (NH4Cl, nigericin, and monensin). Cellular partitioning of propranolol and imipramine was markedly reduced (by at least 40%) by NH4Cl, nigericin, or monensin. Lysosomotropic drugs also inhibited the partitioning of propranolol by at least 50%, with imipramine partitioning affected to a lesser degree. This study demonstrates the usefulness of immortalized hepatocytes (Fa2N-4 cells) for determining the lysosomal sequestration of lipophilic amines.

Phospholipogenic pharmaceuticals are associated with a higher incidence of histological findings than nonphospholipogenic pharmaceuticals in preclinical toxicology studies

While phospholipidosis is thought to be an adaptive response to chemical challenge, many phospholipogenic compounds are known to display adverse effects in preclinical species and humans. To investigate the link between phospholipogenic administration and incidence of preclinical histological signals, an internal AstraZeneca in vivo toxicology report database was searched to identify phospholipogenic and nonphospholipogenic compounds. The datasets assembled comprised 46 phospholipogenic and 62 nonphospholipogenic compounds. The phospholipogenic potential of these compounds was confirmed by a pathologist's interpretation and was supported by well-validated in silico and in vitro models. The phospholipogenic dataset contained 37 bases, 4 neutral compounds, 3 zwitterions, and 1 acid, whereas the nonphospholipogenic dataset contained 9 bases, 34 neutrals, 1 zwitterion, and 18 acids. Histologic findings were tracked for adrenal gland; bone marrow; kidney; liver; lung; lymph node; spleen; thymus; and reproductive organs. On average, plasma exposures were higher in animals dosed with the nonphospholipogenics. Phospholipogenics yielded proportionally more histologic changes (exclusive of phospholipidosis itself) in all organs. Statistically significant higher frequencies of liver necrosis, alveolitis/pneumonitis, as well as lymphocytolysis in the thymus, lymph nodes, and spleen occurred in response to phospholipogenics compared to that for nonphospholipogenics.

Drug-drug interactions involving lysosomes: mechanisms and potential clinical implications

INTRODUCTION

Many commercially available, weakly basic drugs have been shown to be lysosomotropic, meaning they are subject to extensive sequestration in lysosomes through an ion trapping-type mechanism. The extent of lysosomal trapping of a drug is an important therapeutic consideration because it can influence both activity and pharmacokinetic disposition. The administration of certain drugs can alter lysosomes such that their accumulation capacity for co-administered and/or secondarily administered drugs is altered.

AREAS COVERED

In this review the authors explore what is known regarding the mechanistic basis for drug-drug interactions involving lysosomes. Specifically, the authors address the influence of drugs on lysosomal pH, volume and lipid processing.

EXPERT OPINION

Many drugs are known to extensively accumulate in lysosomes and significantly alter their structure and function; however, the therapeutic and toxicological implications of this remain controversial. The authors propose that drug-drug interactions involving lysosomes represent an important potential source of variability in drug activity and pharmacokinetics. Most evaluations of drug-drug interactions involving lysosomes have been performed in cultured cells and isolated tissues. More comprehensive in vivo evaluations are needed to fully explore the impact of this drug-drug interaction pathway on therapeutic outcomes.

Cationic amphiphilic drugs cause a marked expansion of apparent lysosomal volume: implications for an intracellular distribution-based drug interaction

How a drug distributes within highly compartmentalized mammalian cells can affect both the activity and pharmacokinetic behavior. Many commercially available drugs are considered to be lysosomotropic, meaning they are extensively sequestered in lysosomes by an ion trapping-type mechanism. Lysosomotropic drugs typically have a very large apparent volume of distribution and a prolonged half-life in vivo, despite minimal association with adipose tissue. In this report we tested the prediction that the accumulation of one drug (perpetrator) in lysosomes could influence the accumulation of a secondarily administered one (victim), resulting in an intracellular distribution-based drug interaction. To test this hypothesis cells were exposed to nine different hydrophobic amine-containing drugs, which included imipramine, chlorpromazine and amiodarone, at a 10 μM concentration for 24 to 48 h. After exposure to the perpetrators the cellular accumulation of LysoTracker Red (LTR), a model lysosomotropic probe, was evaluated both quantitatively and microscopically. We found that all of the tested perpetrators caused a significant increase in the cellular accumulation of LTR. Exposure of cells to imipramine caused an increase in the cellular accumulation of other lysosomotropic probes and drugs including LyosTracker Green, daunorubicin, propranolol and methylamine; however, imipramine did not alter the cellular accumulation of non-lysosomotropic amine-containing molecules including MitoTracker Red and sulforhodamine 101. In studies using ionophores to abolish intracellular pH gradients we were able to resolve ion trapping-based cellular accumulation from residual pH-gradient independent accumulation. Results from these evaluations in conjunction with lysosomal pH measurements enabled us to estimate the relative aqueous volume of lysosomes of cells before and after imipramine treatment. Our results suggest that imipramine exposure caused a 4-fold expansion in the lysosomal volume, which provides the basis for the observed drug interaction. The imipramine-induced lysosomal volume expansion was shown to be both time- and temperature-dependent and reversed by exposing cells to hydroxypropyl-β-cyclodextrin, which reduced lysosomal cholesterol burden. This suggests that the expansion of lysosomal volume occurs secondary to perpetrator-induced elevations in lysosomal cholesterol content. In support of this claim, the cellular accumulation of LTR was shown to be higher in cells isolated from patients with Niemann-Pick type C disease, which are known to hyperaccumulate cholesterol in lysosomes.

Mechanisms of amine accumulation in, and egress from, lysosomes

The human body is continuously exposed to small organic molecules containing one or more basic nitrogen atoms. Many of these are endogenous (i.e., neurotransmitters, polyamines and biogenic amines), while others are exogenously supplied in the form of drugs, foods and pollutants. It is well-known that many amines have a strong propensity to specifically and substantially accumulate in highly acidic intracellular compartments, such as lysosomes, through a mechanism referred to as ion trapping. It is also known that cells have acquired the unique ability to sense and respond to amine accumulation in lysosomes in an effort to prevent potential negative consequences associated with hyperaccumulation. We describe here methods that are used to evaluate the dynamics of amine accumulation in, and egress from, lysosomes. Moreover, we highlight specific proteins that are thought to play important roles in these pathways. A theoretical model describing lysosomal amine dynamics is described and shown to adequately fit experimental kinetic data. The implications of this research in understanding and treating disease are discussed.

A high content screening assay for identifying lysosomotropic compounds

Lysosomes are acidic organelles that are essential for the degradation of old organelles and engulfed microbes. Furthermore, lysosomes play a key role in cell death. Lipophilic or amphiphilic compounds with a basic moiety can become protonated and trapped within lysosomes, causing lysosomal dysfunction. Therefore, high-throughput screens to detect lysosomotropism, the accumulation of compounds in lysosomes, are desirable. Hence, we developed a 96-well format, high content screening assay that measures lysosomotropism and cytotoxicity by quantitative image analysis. Forty drugs, including antidepressants, antipsychotics, antiarrhythmics and anticancer agents, were tested for their effects on lysosomotropism and cytotoxicity in H9c2 cells. The assay correctly identified drugs known to cause lysosomotropism and revealed novel information showing that the anticancer drugs, gefitinib, lapatinib, and dasatinib, caused lysosomotropism. Although structurally and pharmacologically diverse, drugs that were lysosomotropic shared certain physicochemical properties, possessing a ClogP>2 and a basic pKa between 6.5 and 11. In contrast, drugs which did not lie in this physicochemical property space were not lysosomotropic. The assay is a robust, rapid screen that can be used to identify lysosomotropic, as well as, cytotoxic compounds, and can be positioned within a screening paradigm to understand the role of lysosomotropism as a contributor to drug-induced toxicity.

Screening for phospholipidosis induced by central nervous drugs: comparing the predictivity of an in vitro assay to high throughput in silico assays

Drug-induced phospholipidosis is a side effect for which drug candidates can be screened in the drug discovery phase. The numerous in silico models that have been developed as a first line of screening are based on the characteristic physicochemical properties of phospholipidosis-inducing drugs, e.g. high logP and pK(b) values. However, applying these models on a predominantly high lipophilic, basic CNS chemistry results in a high false positive rate and consequently in a wrong classification of a large number of valuable drug candidates. Here, we tested 33 CNS-compounds (24 in vivo negative and 9 in vivo positive phospholipidosis-inducers) in our in house developed in vitro phospholipidosis screening assay (Mesens et al., 2009) and compared its predictivity with the outcome of three different, well established in silico prediction models. Our in vitro assay demonstrates an increased specificity of 79% over the in silico models (29%). Moreover, by considering the proposed plasma concentration at the efficacious dose we can show a clear correlation between the in vitro and in vivo occurrence of phospholipidosis, improving the specificity of prediction to 96%. Through its high predictive value, the in vitro low throughput assay is thus preferred above high throughput in silico assays, characterized by a high false positive rate.

The TB laboratory of the future: macrophage-based selection of XDR-TB therapeutics

Therapy of multidrug-resistant (MDR)-TB is highly problematic; that of extensively drug-resistant (XDR)-TB even more so. Both infections result in high mortality, especially if the patient is coinfected with HIV or presents with AIDS. Selection of therapy for these infections is limited and, for most situations, it is performed ‘blind’. However, there is a solution for the selection of effective therapy and this is presented herein. Ideal therapy of the patient infected with MDR-TB or XDR-TB can be determined a priori by the mycobacteriology laboratory. This would involve the isolation of the patient’s macrophages, the phagocytosis of the mycobacterial isolate and the presentation of the antitubercular agent to the macrophage–bacterium complex. This system is reviewed in its entirety and its potential and feasibility are supported by hard experimental demonstrations.

Lysosomal sequestration of amine-containing drugs: analysis and therapeutic implications

Amine-containing drugs represent a very important class of therapeutic agents, with the majority of all drugs containing at least one basic nitrogen. For many decades, it has been known that weakly basic compounds can be sequestered into acidic organelles such as lysosomes. Some amines can achieve very high concentrations and induce a dramatic expansion (vacuolization) of the compartment. In the early 70s, Nobel laureate and discoverer of lysosomes, Christian de Duve et al. wrote an elegant commentary describing the theoretical basis for lysosomal sequestration of amines, referring to the process as pH-partitioning and the substrates as lysosomotropics. Recently, a resurgence of interest in the intracellular distribution of drugs has occurred considering its therapeutic importance. Specifically, lysosomal sequestration of amines has received considerable attention for reasons including its involvement in drug resistance, inducement of phospholipidosis, and its influence on whole body distribution/pharmacokinetics. Moreover, the sequestration phenomenon has been recently exploited in the development of a novel drug targeting strategy. This review will focus on these occurrences/developments and conclude with a commentary on the expected impact that knowledge regarding the intracellular distribution of drugs will likely have on future drug development processes.

Enhanced killing of intracellular multidrug-resistant Mycobacterium tuberculosis by compounds that affect the activity of efflux pumps

Whereas human neutrophils are effective and efficient killers of bacteria, macrophages such as those derived from monocytes are almost devoid of killing activity. Nevertheless, monocytes can be transformed into effective killers of mycobacteria or staphylococci when exposed to clinical concentrations of a phenothiazine or to inhibitors of efflux pumps (reserpine and verapamil), or to ouabain, an inhibitor of K(+) transport. Because the rates of multidrug-resistant Mycobacterium tuberculosis (MDR-TB) continue to escalate globally, and because no new effective drug has been made available for almost 40 years, compounds that enhance the killing activity of monocytes against MDR-TB are obviously needed. This review covers the specific characteristics of MDR-TB, identifies a variety of agents that address these characteristics and therefore have potential for managing MDR-TB. Because the mechanism by which these agents enhance the killing of intracellular bacteria is important for the intelligent design of new anti-tubercular agents, the review correlates the mechanisms by which these agents manifest their effects. Lastly, a model is presented which describes the mechanisms by which distinct efflux pumps of the phagosome-lysosome complex are inhibited by agents that are known to inhibit K(+) flux. The model also predicts the existence of a K(+) activated exchange (pump) that is probably located in the membrane that delineates the lysosome. This putative pump, which is immune to inhibitors of K+ flux, is identified as being the cause for the acidification of the lysosome thereby activating its hydrolytic enzymes. Because the non-killer macrophage can be transformed into an effective killer by a variety of compounds that inhibit K(+) transport, perhaps it would be wise to develop drugs that enhance the killing activity of these cells inasmuch as this approach would not be subject to any resistance, as is the eventual case for conventional antibiotics.

Lysosomotropism of Basic Cathepsin K Inhibitors Contributes to Increased Cellular Potencies against Off-Target Cathepsins and Reduced Functional Selectivity

The lysosomal cysteine protease cathepsin K is a target for osteoporosis therapy. The aryl-piperazine-containing cathepsin K inhibitor CRA-013783/L-006235 (1) displays greater than 4000-fold selectivity against the lysosomal/endosomal antitargets cathepsin B, L, and S. However, 1 and other aryl-piperazine-containing analogues, including balicatib (10), are approximately 10-100-fold more potent in cell-based enzyme occupancy assays than against each purified enzyme. This phenomenon arises from their basic, lipophilic nature, which results in lysosomal trapping. Consistent with its lysosomotropic nature, 1 accumulates in cells and in rat tissues of high lysosome content. In contrast, nonbasic aryl-morpholino-containing analogues do not exhibit lysosomotropic properties. Increased off-target activities of basic cathepsin K inhibitors were observed in a cell-based cathepsin S antigen presentation assay. No potency increases of basic inhibitors in a functional cathepsin K bone resorption whole cell assay were detected. Therefore, basic cathepsin K inhibitors, such as 1, suffer from reduced functional selectivities compared to those predicted using purified enzyme assays.

Quantitation of the lysosomotropic character of cationic amphiphilic drugs using the fluorescent basic amine Red DND-99

Cationic amphiphilic drugs (CAD) containing a basic moiety often accumulate in lysosomes or other acidic subcellular compartments. This lysosomotropism is due to the protonation of the CAD within acidic organelles leading to the formation of a membrane-impermeable form. Highly lipophilic CADs show a greater propensity to accumulate than those with a lower lipophilicity (Log P). Here we describe a rapid method to quantitate the uptake of CAD within lysosomes. The principle of the method involves a competition between the CAD and the fluorescent basic amine probe Red DND-99 (LysoTracker) in HeLa cells. Visualization is performed using confocal fluorescence microscopy. Loading HeLa cells with 10-150 nMRed DND-99 gives a punctuated fluorescent pattern around the cell nucleus. This fluorescence is displaced by incubating the cells with 10-100mM NH(4)Cl, either before or after the addition of the probe, consistent with the reversible partitioning of the probe in the lysosome. The displacement of probe fluorescence by CAD such as chlorpromazine and imipramine was observed in a dose-dependent manner. Lower concentrations of CAD with high Log P displaced the probe more efficiently than those with a lower Log P. This assay represents a rapid and semiquantitative method for the measurement of lysosomotropism in an in vitro system without the use of radioactivity and cell fractionation. Furthermore, the microscopy confirms the targeting of the basic compounds to within the lysosomes.

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.

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.

Prediction of pharmacokinetics prior to in vivo studies. 1. Mechanism-based prediction of volume of distribution

In drug discovery and nonclinical development the volume of distribution at steady state (V(ss)) of each novel drug candidate is commonly determined under in vivo conditions. Therefore, it is of interest to predict V(ss) without conducting in vivo studies. The traditional description of V(ss) corresponds to the sum of the products of each tissue:plasma partition coefficient (P(t:p)) and the respective tissue volume in addition to the plasma volume. Because data on volumes of tissues and plasma are available in the literature for mammals, the other input parameters needed to estimate V(ss) are the P(t:p)'s, which can potentially be predicted with established tissue composition-based equations. In vitro data on drug lipophilicity and plasma protein binding are the input parameters used in these equations. Such a mechanism-based approach would be particularly useful to provide first-cut estimates of V(ss) prior to any in vivo studies and to explore potential unexpected deviations between sets of predicted and in vivo V(ss) data, when the in vivo data become available during the drug development process. The objective of the present study was to use tissue composition-based equations to predict rat and human V(ss) prior to in vivo studies for 123 structurally unrelated compounds (acids, bases, and neutrals). The predicted data were compared with in vivo data obtained from the literature or at Roche. Overall, the average ratio of predicted-to-experimental rat and human V(ss) values was 1.06 (SD = 0.817, r = 0.78, n = 147). In fact, 80% of all predicted values were within a factor of two of the corresponding experimental values. The drugs can therefore be separated into two groups. The first group contains 98 drugs for which the predicted V(ss) were within a factor of two of those experimentally determined (average ratio of 1.01, SD = 0.39, r = 0.93, n = 118), and the second group includes 25 other drugs for which the predicted and experimental V(ss) differ by a factor larger than two (average ratio of 1.32, SD = 1.74, r = 0.42, n = 29). Thus, additional relevant distribution processes were neglected in predicting V(ss) of drugs of the second group. This was true especially in the case of some cationic-amphiphilic bases. The present study is the first attempt to develop and validate a mechanistic distribution model for predicting rat and human V(ss) of drugs prior to in vivo studies.

Phenothiazines: potential management of Creutzfeldt-Jacob disease and its variants

Creutzfeldt-Jakob disease acquired from bovines (nvCJD) has been responsible for nearly 100 deaths in the UK and thousands more may die in the years to come. New variant CJD (nvCJD) is incurable and although clinical diagnosis is becoming more precise, the diagnosis is only certain at autopsy. Phenothiazine derivatives inhibit production of prions, the disease causing agent, in cultured neuroblastoma cells, and an advanced case of nvCJD was recently brought to remission by the use of these agents in combination with an antimalarial. In this review we present direct and circumstantial evidence in support of a model describing the manner by which the intracellular antimicrobial activity of phenothiazines might cause the destruction of intracellular prions.

Intracellular distribution of psychotropic drugs in the grey and white matter of the brain: the role of lysosomal trapping

1. Since the brain is not a homogenous organ (i.e. the phospholipid pattern and density of lysosomes may vary in its different regions), in the present study we examined the uptake of psychotropic drugs by vertically cut slices of whole brain, grey (cerebral cortex) and white (corpus callosum, internal capsule) matter of the brain and by neuronal and astroglial cell cultures. 2. Moreover, we assessed the contribution of lysosomal trapping to total drug uptake (total uptake=lysosomal trapping+phospholipid binding) by tissue slices or cells conducting experiments in the presence and absence of 'lysosomal inhibitors', i.e., the lysosomotropic compound ammonium chloride (20 mM) or the Na(+)/H(+)-ionophore monensin (10 microM), which elevated the internal pH of lysosomes. The initial concentration of psychotropic drug in the incubation medium was 5 microM. 3. Both total uptake and lysosomal trapping of the antidepressants investigated (imipramine, amitriptyline, fluoxetine, sertraline) and neuroleptics (promazine, perazine, thioridazine) were higher in the grey matter and neurones than in the white matter and astrocytes, respectively. Lysosomal trapping of the psychotropics occurred mainly in neurones where thioridazine sertraline and perazine showed the highest degree of lysosomotropism. 4. Distribution interactions between antidepressants and neuroleptics took place in neurones via mutual inhibition of lysosomal trapping of drugs. 5. A differential number of neuronal and glial cells in the brain may mask the lysosomal trapping and the distribution interactions of less potent lysosomotropic drugs in vertically cut brain slices. 6. A reduction (via a distribution interaction) in the concentration of psychotropics in lysosomes (depot), which leads to an increase in their level in membranes and tissue fluids, may intensify the pharmacological action of the combined drugs.

Pharmacokinetics and metabolism of thioridazine during co-administration of tricyclic antidepressants

1. Because of serious side-effects of thioridazine and tricyclic antidepressants (cardiotoxicity), a possible influence of imipramine and amitriptyline on the pharmacokinetics and metabolism of thioridazine was investigated in a steady state (2-week treatment) in rats. 2. Imipramine and amitriptyline (5 and 10 mg kg(-1) i.p., respectively) elevated 30 and 20 fold, respectively, the concentration of thioridazine (10 mg kg(-1) i.p.) and its metabolites (N-desmethylthioridazine, 2-sulphoxide, 2-sulphone, 5-sulphoxide) in blood plasma. Similar, yet weaker increases in the thioridazine concentration were found in the brain. Moreover, an elevation of thioridazine/metabolite ratios was observed. 3. Imipramine and amitriptyline added to control liver microsomes in vitro inhibited the metabolism of thioridazine via N-demethylation (an increase in K(m)), mono-2-sulphoxidation (an increase in K(m) and a decrease in V(max)) and 5-sulphoxidation (mainly a decrease in V(max)). Amitriptyline was a more potent inhibitor than imipramine of the thioridazine metabolism. 4. The varying concentration ratios of antidepressant/thioridazine in vivo appear to be more important to the final result of the pharmacokinetic interactions than are relative direct inhibitory effects of the antidepressants on thioridazine metabolism observed in vitro. 5. Besides direct inhibition of the thioridazine metabolism, the decreased activity of cytochrome P-450 towards 5-sulphoxidation, produced by chronic joint administration of thioridazine and the antidepressants, seems to be relevant to the observed in vivo interaction. 6. The obtained results may also point to inhibition of another, not yet investigated, metabolic pathway of thioridazine, which may be inferred from the simultaneous elevation of concentrations of both thioridazine and the measured metabolites.