The antimalarials quinacrine and chloroquine induce weak lysosomal storage of sulphated glycosaminoglycans in cell culture and in vivo

Drugs affecting glycosaminoglycan metabolism

Glycosaminoglycans (GAGs) are charged polysaccharides ubiquitously present at the cell surface and in the extracellular matrix. GAGs are crucial for cellular homeostasis, and their metabolism is altered during pathological processes. However, little consideration has been given to the regulation of the GAG milieu through pharmacological interventions. In this review, we provide a classification of small molecules affecting GAG metabolism based on their mechanism of action. Furthermore, we present evidence to show that clinically approved drugs affect GAG metabolism and that this could contribute to their therapeutic benefit.

Therapy and pharmacological properties of hydroxychloroquine and chloroquine in treatment of systemic lupus erythematosus, rheumatoid arthritis and related diseases

OBJECTIVES

This review examines the pharmacokinetics, modes of action and therapeutic properties of the anti-malarial drugs, hydroxychloroquine (HCQ) and chloroquine (CQ), in the treatment of systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and related conditions, as well as osteoarthritis (OA).

KEY FINDINGS

Both HCQ and CQ have historically been employed successfully for the treatment of SLE and RA for over 70 years. HCQ has been used extensively for SLE where it has a good reputation for controlling the dermatological complications in SLE. It has also been reported to effectively control the symptoms of Sjøgren's syndrome, as well as preventing thrombosis in phospholipid antibody (aPL) syndrome. In RA and SLE, HCQ is preferred because of the lower incidence of gastrointestinal adverse reactions compared with CQ and it might have a lower risk of ocular adverse reactions. There is increasing evidence that HCQ may reduce atherosclerosis and risks of cardiovascular disease in rheumatic patients. Both HCQ and CQ have been shown to improve glycaemia and reduce the risks of type II diabetes mellitus. Although both HCQ and CQ are effective in low-moderate RA, HCQ is now preferred as part of combination therapy for more severe disease. The advantages of combination therapy are that the doses of the individual drugs may be lowered so reducing adverse reactions. Both HCQ and CQ are diastereoisomers, have basic properties and are given as the sulphate and phosphate salts. While being relatively well absorbed orally and with good bioavailability, they have long and variable plasma terminal elimination half-lives (approximately 40-60 days). This reflects their high volume of distribution, V D (HCQ 44,000L; CQ 65,000L) which extends into aqueous compartments, long mean residence time (HCQ 1300 h; CQ 900 h) and with about half the drugs (metabolites) undergoing renal clearance. The strong binding to melanin reflects the ocular injury and dermatological properties of these drugs. The consensus is that the occurrence of ocular adverse reactions can be minimised by close attention to the dose (which should be set on a body weight basis) with regular (e.g. quarterly) retinal examination. Although HCQ and CQ can pass through the placenta, the use of these drugs during pregnancy does not appear to risk harm to the baby and might be beneficial to the mother with SLE and her child by controlling the SLE disease activity, which is known to be an important factor affecting pregnancy outcome. The modes of action of HCQ and CQ in these arthritides represent somewhat of an enigma. Undoubtedly, these drugs have multiple actions related, in part, their ability to accumulate in lysosomes and autophagosomes of phagocytic cells as well as affecting MHC Class II expression and antigen presentation; actions of the production of pro-inflammatory cytokines [e.g. interleukin-1 (IL-1) tumour necrosis factor-α (TNFα)]; control of toll-like receptor-9 activation; and leucocyte generation of reactive oxygen species (ROS); i.e. antioxidant activity. The actions of these drugs on T and B cells are less clear but may depend on these leucocyte-mediated actions. Anti-malarials also protect against cytokine-mediated cartilage resorption. This and other actions may underlie the potential benefits in treating OA. The exact relationships of these various actions, mostly determined in vitro, have not been specifically defined in vivo or ex vivo in relation to clinical efficacy.

OUTCOMES

HCQ and CQ have a good reputation for being effective and relatively safe treatments in SLE, mild-moderate RA and Sjøgren's syndrome. There is need for (a) more information on their mode of action in relation to the control of these diseases, (b) scope for developing formulations that have improved pharmacokinetic and therapeutic properties and safety, and (c) further exploring their use in drug combinations not only with other disease modifying agents but also with biologics.

The Fab1/PIKfyve phosphoinositide phosphate kinase is not necessary to maintain the pH of lysosomes and of the yeast vacuole

Lysosomes and the yeast vacuole are degradative and acidic organelles. Phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2), a master architect of endolysosome and vacuole identity, is thought to be necessary for vacuolar acidification in yeast. There is also evidence that PtdIns(3,5)P2 may play a role in lysosomal acidification in higher eukaryotes. Nevertheless, these conclusions rely on qualitative assays of lysosome/vacuole pH. For example, quinacrine, an acidotropic fluorescent base, does not accumulate in the vacuoles of fab1Δ yeast. Fab1, along with its mammalian ortholog PIKfyve, is the lipid kinase responsible for synthesizing PtdIns(3,5)P2. In this study, we employed several assays that quantitatively assessed the lysosomal and vacuolar pH in PtdIns(3,5)P2-depleted cells. Using ratiometric imaging, we conclude that lysosomes retain a pH < 5 in PIKfyve-inhibited mammalian cells. In addition, quantitative fluorescence microscopy of vacuole-targeted pHluorin, a pH-sensitive GFP variant, indicates that fab1Δ vacuoles are as acidic as wild-type yeast. Importantly, we also employed fluorimetry of vacuoles loaded with cDCFDA, a pH-sensitive dye, to show that both wild-type and fab1Δ vacuoles have a pH < 5.0. In comparison, the vacuolar pH of the V-ATPase mutant vph1Δ or vph1Δ fab1Δ double mutant was 6.1. Although the steady-state vacuolar pH is not affected by PtdIns(3,5)P2 depletion, it may have a role in stabilizing the vacuolar pH during salt shock. Overall, we propose a model in which PtdIns(3,5)P2 does not govern the steady-state pH of vacuoles or lysosomes.

In silico studies and fluorescence binding assays of potential anti-prion compounds reveal an important binding site for prion inhibition from PrP(C) to PrP(Sc)

To understand the pharmacophore properties of 2-aminothiazoles and design novel inhibitors against the prion protein, a highly predictive 3D quantitative structure-activity relationship (QSAR) has been developed by performing comparative molecular field analysis (CoMFA) and comparative similarity analysis (CoMSIA). Both CoMFA and CoMSIA maps reveal the presence of the oxymethyl groups in meta and para positions on the phenyl ring of compound 17 (N-[4-(3,4-dimethoxyphenyl)-1,3-thiazol-2-yl]quinolin-2-amine), is necessary for activity while electro-negative nitrogen of quinoline is highly favorable to enhance activity. The blind docking results for these compounds show that the compound with quinoline binds with higher affinity than isoquinoline and naphthalene groups. Out of 150 novel compounds retrieved using finger print analysis by pharmacophoric model predicted based on five test sets of compounds, five compounds with diverse scaffolds were selected for biological evaluation as possible PrP inhibitors. Molecular docking combined with fluorescence quenching studies show that these compounds bind to pocket-D of SHaPrP near Trp145. The new antiprion compounds 3 and 6, which bind with the interaction energies of -12.1 and -13.2 kcal/mol, respectively, show fluorescence quenching with binding constant (Kd) values of 15.5 and 44.14 μM, respectively. Further fluorescence binding assays with compound 5, which is similar to 2-aminothiazole as a positive control, also show that the molecule binds to the pocket-D with the binding constant (Kd) value of 84.7 μM. Finally, both molecular docking and a fluorescence binding assay of noscapine as a negative control reveals the same binding site on the surface of pocket-A near a rigid loop between β2 and α2 interacting with Arg164. This high level of correlation between molecular docking and fluorescence quenching studies confirm that these five compounds are likely to act as inhibitors for prion propagation while noscapine might act as a prion accelerator from PrP(C) to PrP(Sc).

Small molecule inhibitors of protein interaction with glycosaminoglycans (SMIGs), a novel class of bioactive agents with anti-inflammatory properties

BACKGROUND

Small molecule inhibitors of biologically important protein-glycosaminoglycan (GAG) interactions have yet to be identified.

METHODS

Compound libraries were screened in an assay of L-selectin-IgG binding to heparin (a species of heparan sulfate [HS-GAG]). Hits were validated, IC-50s established and direct binding of hits to HS-GAGs was investigated by incubating compounds alone with heparin. Selectivity of inhibitors was assessed in 11 different protein-GAG binding assays. Anti-inflammatory activity of selected compounds was evaluated in animal models.

RESULTS

Screening identified a number of structurally-diverse planar aromatic cationic amines. Scaffolds similar to known GAG binders, chloroquine and tilorone, were also identified. Inhibitors displayed activity also against bovine kidney heparan sulfate. Direct binding of compounds to GAGs was verified by incubating compounds with heparin alone. Selectivity of inhibitors was demonstrated in a panel of 11 heparin binding proteins, including selectins, chemokines (IL-8, IP-10), Beta Amyloid and cytokines (VEGF, IL-6). A number of selected lead compounds showed dose-dependent efficacy in peritonitis, paw edema and delayed type hypersensitivity.

CONCLUSIONS

A new class of compounds, SMIGs, inhibits protein-GAG interaction by direct binding to GAGs. Although their IC-50s were in the low micro-molar range, SMIGs binding to HS-GAGs appeared to be stable in physiological conditions, indicating high avidity binding. SMIGs may interfere with major checkpoints for inflammatory and autoimmune events.

GENERAL SIGNIFICANCE

SMIGs are a class of structurally-diverse planar aromatic cationic amines that have an unusual mode of action - inhibiting protein-GAG interactions via direct and stable binding to GAGs. SMIGs may have therapeutic potential in inflammatory and autoimmune disorders.

Discovery of carmegliptin: a potent and long-acting dipeptidyl peptidase IV inhibitor for the treatment of type 2 diabetes

Design, synthesis, and SAR are described for a class of DPP-IV inhibitors based on aminobenzo[a]quinolizines with non-aromatic substituents in the S1 specificity pocket. One representative thereof, carmegliptin (8p), was chosen for clinical development. Its X-ray structure in complex with the enzyme and early efficacy data in animal models of type 2 diabetes are also presented.

Chemically induced accumulation of GAGs delays PrP(Sc) clearance but prolongs prion disease incubation time

Prion diseases are a group of fatal neurodegenerative diseases affecting humans and animals. The only identified component of the infectious prion is PrP(Sc), an aberrantly folded isoform of PrP(C). Glycosaminoglycans, which constitute the main receptor for prions on cells, play a complex role in the pathogenesis of prion diseases. For example, while agents inducing aberrant lysosomal accumulation of GAGs such as Tilorone and Quinacrine significantly reduced PrP(Sc) content in scrapie-infected cells, administration of Quinacrine to prion-infected subjects did not improve their clinical status. In this study, we investigated the association of PrP(Sc )with cells cultured with Tilorone. We found that while the initial incorporation of PrP(Sc) was similar in the treated and untreated cells, clearance of PrP(Sc) from the Tilorone-treated cells was significantly impaired. Interestingly, prolonged administration of Tilorone to mice prior to prion infection resulted in a significant delay in disease onset, concomitantly with in vivo accumulation of lysosomal GAGs. We hypothesize that GAGs may complex with newly incorporated PrP(Sc) in lysosomes and further stabilize the prion protein conformation. Over-stabilized PrP(Sc) molecules have been shown to comprise reduced converting activity.

Synthesis and SAR study of acridine, 2-methylquinoline and 2-phenylquinazoline analogues as anti-prion agents

Transmissible spongiform encephalopathies (TSEs) are thought to arise from aggregation of a protease resistant protein denoted PrP(Sc), which is a misfolded isoform of the normal cellular prion protein PrP(C). Using virtual high-throughput screening we have selected structures analogous to acridine, 2-methyquinoline and 2-phenylquinazoline as potential therapeutic candidates for the treatment of TSEs. From the synthesis and screening of constructed libraries we have shown that an electron-rich aromatic ring attached through an amine linker to the position para to the ring nitrogen is beneficial to both binding to PrP(C) and the suppression of PrP(Sc) accumulation for acridine and 2-methylquinoline analogues. 2-Phenylquinazoline analogues appear to utilise a different mode of action by binding at a different location and/or pose. We report IC50s in the nanomolar range.

Binding of anti-prion agents to glycosaminoglycans: Evidence from electronic absorption and circular dichroism spectroscopy

The polyanionic glycosaminoglycans (GAGs) are intimately involved in the pathogenesis of protein conformational disorders such as amyloidosis and prion diseases. Several cationic agents are known to exhibit anti-prion activity but their mechanism of action is poorly understood. In this study, UV absorption and circular dichroism (CD) spectroscopic techniques were used to investigate the interaction between heparin and chondroitin-6-sulfate and anti-prion drugs including acridine, quinoline, and phenothiazine derivatives. UV band hypochromism of (+/-)-quinacrine, (+/-)-primaquine, tacrine, quinidine, chlorpromazine, and induced CD spectra of (+/-)-quinacrine upon addition of GAGs provided evidence for the GAG binding of these compounds. The association constants (approximately 10(6)-10(7)M(-1)) estimated from the UV titration curves show high-affinity drug-heparin interactions. Ionic strength-dependence of the absorption spectra suggested that the interaction between GAGs and the cationic drugs is principally electrostatic in nature. Drug binding differences of heparin and chondroitin-6-sulfate were attributed to their different negative charge density. These results call the attention to the alteration of GAG-prion/GAG-amyloid interactions by which these compounds might exert their anti-prion/anti-amyloidogenic activities.

Synthesis and biological evaluation of novel hexahydro-pyrido[3′,2′:4,5]pyrrolo[1,2-a]pyrazines as potent and selective 5-HT2C receptor agonists

Further lead optimization efforts on previously described 1,2,3,4,10,10a-hexahydro-1H-pyrazino[1,2-a]indoles led to the new class of 5,5a,6,7,8,9-hexahydro-pyrido[3',2':4,5]pyrrolo[1,2-a]pyrazines culminating in the discovery of (5aR,9R)-2-[(cyclopropylmethoxy)methyl]-5,5a,6,7,8,9-hexahydro-9-methyl-pyrido[3', 2':4,5]pyrrolo[1,2-a]pyrazine 18 as a potent, full 5-HT(2C) receptor agonist with an outstanding selectivity profile and excellent hERG and phospholipidosis properties.

Ecotoxicological evaluation of the antimalarial drug chloroquine

There is limited information available about the potential environmental effects of chloroquine (CQ), a widely used antimalarial agent and a promising inexpensive drug in the management of HIV disease. The acute effects of CQ were studied using four ecotoxicological model systems. The most sensitive bioindicator was the immobilization of the cladoceran Daphnia magna, with an EC50 of 12 microM CQ at 72 h and a non-observed adverse effect level of 2.5 microM CQ, followed very closely by the decrease of the uptake of neutral red and the reduction of the lysosomal function in the fish cell line PLHC-1 derived from the top minnow Poeciliopsis lucida, probably due to the selective accumulation of the drug into the lysosomes. There was significant cellular stress as indicated by the increases on metallothionein and glucose-6P dehydrogenase levels after 24 h of exposure and succinate dehydrogenase activity mainly after 48 h. No changes were observed for ethoxyresorufin-O-deethylase (EROD) activity. The least sensitive model was the inhibition of bioluminescence in the bacterium Vibrio fischeri. An increase of more than five-fold in the toxicity from 24 to 72 h of exposure was observed for the inhibition of the growth in the alga Chlorella vulgaris and the content of total protein and MTS tetrazolium salt metabolization in PLHC-1 cells. At the morphological level, the most evident alterations in PLHC-1 cultures were hydropic degeneration from 25 microM CQ after 24h of exposure and the presence of many cells with pyknotic nuclei, condensed cytoplasm and apoptosis with concentrations higher than 50 microM CQ after 48 h of exposure. In conclusion, CQ should be classified as harmful to aquatic organisms.

An aminomethylpyrimidine DPP-IV inhibitor with improved properties

A recently identified DPP-IV inhibitor (1) was found to induce phospholipidosis and to inhibit CYP3A4. A small series of less lipophilic and less amphiphilic analogues was synthesized in an effort to overcome these issues. One compound from this series was equipotent to 1, did not induce phospholipidosis and showed a reduced CYP3A4 inhibition.

Bull's-eye maculopathy associated with quinacrine therapy for malaria

PURPOSE

To report a side effect of quinacrine therapy for malaria.

DESIGN

Case report.

METHODS

Review of clinical chart and photographs.

SETTING

Private retina practice.

RESULTS

A patient developed a bilaterally symmetric bull's-eye maculopathy 45 years after taking quinacrine for 18 months as prophylaxis against malaria. Progression of the clinical picture was documented over 15 years of follow-up. The clinical picture was identical to that of chloroquine and hydroxychloroquine maculopathy.

CONCLUSIONS

Low dosages of quinacrine used for malaria prophylaxis can be associated with a delayed, severe maculopathy indistinguishable from chloroquine maculopathy in certain patients.

Evaluation of quinacrine treatment for prion diseases

Based on in vitro observations in scrapie-infected neuroblastoma cells, quinacrine has recently been proposed as a treatment for Creutzfeldt-Jakob disease (CJD), including a new variant CJD which is linked to contamination of food by the bovine spongiform encephalopathy (BSE) agent. The present study investigated possible mechanisms of action of quinacrine on prions. The ability of quinacrine to interact with and to reduce the protease resistance of PrP peptide aggregates and PrPres of human and animal origin were analyzed, together with its ability to inhibit the in vitro conversion of the normal prion protein (PrPc) to the abnormal form (PrPres). Furthermore, the efficiencies of quinacrine and chlorpromazine, another tricyclic compound, were examined in different in vitro models and in an experimental murine model of BSE. Quinacrine efficiently hampered de novo generation of fibrillogenic prion protein and PrPres accumulation in ScN2a cells. However, it was unable to affect the protease resistance of preexisting PrP fibrils and PrPres from brain homogenates, and a "curing" effect was obtained in ScGT1 cells only after lengthy treatment. In vivo, no detectable effect was observed in the animal model used, consistent with other recent studies and preliminary observations in humans. Despite its ability to cross the blood-brain barrier, the use of quinacrine for the treatment of CJD is questionable, at least as a monotherapy. The multistep experimental approach employed here could be used to test new therapeutic regimes before their use in human trials.

Treatment of scleromyxoedema with hydroxychloroquine

BACKGROUND

Scleromyxoedema is a rare disease of unknown aetiology that is characterized by deposition of mucin and sclerotic induration of the skin; it is associated with paraproteinaemia. Patients suffer from progressive disability due to immobilization and cosmetic disfigurement. Treatment of scleromyxoedema is a therapeutic challenge. The antimalarial hydroxychloroquine has a rapid and reliable effect in reticular erythematous mucinosis.

PATIENTS AND METHODS

Four consecutive patients (two women, two men; median age: 50 years) with scleromyxoedema, three of them with IgG lambda paraprotein, were treated with hydroxychloroquine. Treatment was initiated with 600 mg p.o. for 10 days, followed by 400 mg for at least 4 weeks, and 200 mg thereafter.

RESULTS

Complete remission of skin manifestations was achieved in one patient, whereas three patients achieved a partial remission of 61+, 5 and 25 months' duration. Notably, three patients felt increased mobility and reduced firmness of skin during the first week of treatment, which was reflected in a rapid reduction in dermal thickness. In one patient, dysphagia was reverted as evidenced by normalization of oesophageal clearance. Paraproteinaemia was not influenced at all. Side effects included one case of electroretinogram abnormalities after 19 months of therapy and one case of leucopenia after 3 months.

CONCLUSION

Hydroxychloroquine is an effective form of therapy for scleromyxoedema, leading to rapid and prolonged alleviation of symptoms.

Lysosomotropic agents and cysteine protease inhibitors inhibit scrapie-associated prion protein accumulation

We report that lysosomotropic agents and cysteine protease inhibitors inhibited protease-resistant prion protein accumulation in scrapie-infected neuroblastoma cells. The inhibition occurred without either apparent effects on normal prion protein biosynthesis or turnover or direct interactions with prion protein molecules. The findings introduce two new classes of inhibitors of the formation of protease-resistant prion protein.

Fluid secretion by isolated Malpighian tubules of Drosophila melanogaster Meig.: effects of organic anions, quinacrine and a diuretic factor found in the secreted fluid

Para-aminohippuric acid (PAH, 0.2 and 1 mmol l(−)(1)) had no effect on the basal fluid secretion rate (FSR) of isolated Malpighian tubules of Drosophila melanogaster Meig. and did not affect stimulation of the FSR induced by adenosine 3′,5′-monophosphate (cAMP). Phenol Red (phenolsulphonphthalein, PSP; 0.5 and 1 mmol l(−)(1)) slowed the FSR and abolished stimulation of the FSR by cAMP. Diodrast (1 mmol l(−)(1)) slightly, but significantly, reduced the FSR and greatly reduced the stimulation of the FSR normally provoked by cAMP and by the 3′,5′-monophosphates of guanosine (cGMP), inosine (cIMP) and uridine (cUMP). However, stimulation of the FSR by the 3′, 5′-monophosphate of cytidine (cCMP) was little affected by diodrast. Probenecid (0.2 or 1 mmol l(−)(1)) consistently stimulated the FSR, on average by approximately 25 %, but did not markedly inhibit the subsequent stimulation of the FSR by cAMP, cGMP or cIMP. However, the FSR of tubules stimulated by cGMP was temporarily lowered by probenecid. Quinacrine (0.1 mmol l(−)(1)) slowed basal FSR by an average of approximately 30 %, but subsequent stimulation of the FSR by cAMP was not noticeably affected. Both 0.1 mmol l(−)(1) cAMP and 1 mmol l(−)(1) probenecid stimulated adenylate cyclase activity in extracts of Malpighian tubules, but cIMP, cGMP, cUMP and diodrast were without effect in this regard. Uptake of radioactivity from a solution containing 500 nmol l(−)(1) [(3)H]cAMP and 9.5 μmol l(−)(1) cAMP was reduced by more than 90 % by 1 mmol l(−)(1) PSP, by approximately 40 % by 0.2 mmol l(−)(1) probenecid, by 36 % by 1 mmol l(−)(1) diodrast and by 30 % by 1 mmol l(−)(1) PAH. Neither 0.01 mmol l(−)(1) ouabain nor 0.1 mmol l(−)(1) quinacrine affected the uptake of [(3)H]cAMP by the Malpighian tubules. Fluid secreted by isolated Malpighian tubules of Drosophila melanogaster contains a factor that stimulated the FSR on average by approximately 50 %. The presence in the secreted fluid of cGMP at a concentration of 8.3 μmol l(−)(1) did not explain the stimulatory effect on FSR. These results support the existence of a carrier-mediated uptake of cyclic nucleotides into the Malpighian tubules of Drosophila melanogaster, possibly involving a multispecific transporter.

Lysosomal glycosaminoglycan storage as induced by dicationic amphiphilic drugs: investigation into the mechanisms underlying the slow reversibility

Several dicationic amphiphilic compounds, such as the immunomodulator tilorone and analogues, impair the lysosomal catabolism of sulphated glycosaminoglycans (GAGs). Thereby they cause lysosomal GAG storage in rats and in cultured fibroblasts of several species including man. The GAG storage is rather slowly reversible in vivo; it persists for months after discontinuance of drug treatment. In the present study, we investigated the mechanisms underlying the slow reversibility. Cultured bovine corneal fibroblasts were pretreated for 4 days with tilorone (5 and 20 microM) or with compound CL-90.100 (3 and 10 microM) and further cultured in drug-free medium for periods up to 11 days. The intracellular GAG storage was analysed biochemically and demonstrated histochemically. The subcellular drug distribution (CL-90.100) was demonstrated by fluorescence microscopy. Dermatan sulphate (DS) provided the predominant contribution towards the GAG storage. After pretreatments with the low, as well as the high concentrations of either drug, the storage of DS was irreversible during the period of observation, whereas the minor storage of heparan sulphate was resolved. The enhanced secretion of the lysosomal enzyme beta-hexosaminidase (E.C. 3.2.1.52) caused by pretreatment with the high concentration of tilorone was also readily reversible. Thus, enzyme deprivation could not be the explanation for the sustained DS storage. The localization of the drug-related fluorescence within perinuclear cell organelles, presumably lysosomes, resembled that of the stored GAGs as visualized by histochemical staining. Both, the fluorescence and the positive GAG staining persisted with unchanged intracellular distribution throughout the recovery period. The present results suggest that the persistence of the DS storage is due to the formation of long-lived, non-degradable DS-drug complexes within the lysosomes.

Drug-induced lysosomal storage of sulphated glycosaminoglycans

1. Certain compounds (e.g., the immunomodulator tilorone and congeners) are able to induce lysosomal storage of sulphated glycosaminoglycans (GAG), thus, producing cytological and biochemical alterations reminiscent of the inherited mucopolysaccharidoses. The drug-induced GAG storage has been studied in cultured fibroblasts of several species and in rats, and it is likely to occur also in humans. 2. The cytological hallmarks of GAG storage are enlarged lysosomes congested with material that is intensely stained by cationic dyes. With respect to fixation techniques, one has to keep in mind that the GAGs are highly water-soluble and are leached during conventional fixation and tissue processing. Biochemically, the elevation of GAG contents in tissues and cultured fibroblasts is due to storage of dermatan sulphate, predominantly. 3. The molecular structure of the potent inducers of GAG storage is characterized by a planar tricyclic aromatic ring system that is symmetrically substituted with two side chains of 4-5 sigma bond length, each carrying a protonizable nitrogen atom. The lysosomal storage of GAG is accompanied by lysosomal accumulation of the inducing drug, with the molar ratio of drug to GAG-disaccharide unit amounting to > 1:1. The reversibility of GAG storage is rather slow. 4. The pathogenic mechanisms underlying the drug side effects are discussed and the following hypothesis is put forward: The compounds in question are lysosomotropic weak bases. They get trapped in the acidic lysosomes and accumulate highly there. Physicochemical data suggest that the drugs form complexes with the sulphated GAGs, particularly with dermatan sulphate: The positively charged nitrogen atoms of the drug side chains interact with the negative charges of sulphate and carboxy groups of the GAGs, thereby crosslinking at least two GAG helices. Moreover, the interlinking drug molecules form parallel stacks resulting from interaction of the aromatic pi-electrons of the planar ring systems. This further stabilizes the complexes. The GAGs within the complexes are thought to be resistant to the degrading lysosomal enzymes. 5. Drug-induced GAG storage has not been directly demonstrated in man. Yet, clinical reports on keratopathy and basophilic cytoplasmic inclusions in blood lymphocytes of tilorone-treated patients suggest that this drug side effect may also occur in man.