Volume 2 Supplement 1
Report of a Scientific Working Group on Serious Adverse Events following Mectizan®treatment of onchocerciasis in Loa loa endemic areas
Ivermectin: does P-glycoprotein play a role in neurotoxicity?
© BioMed Central Ltd 2003
Published: 24 October 2003
The macrocyclic lactone ivermectin (Mectizan®) is widely used for the control of human filarial infections, particularly as a donated product for onchocerciasis and lymphatic filariasis. In the case of control of lymphatic filariasis in Africa, it is used in combination with donated albendazole. In areas co-endemic for Onchocerciasis and Loa loa, serious adverse reactions have been observed in patients with apparently high microfilaria counts of Loa loa. Recent findings suggest that the severe central nervous system side effects seen in various vertebrates following ivermectin treatment may be due to an absence of, or functional deficiency in P-glycoprotein. P-glycoprotein is expressed in the apical membrane of brain capillary epithelial cells and is responsible for limiting the brain penetration of a range of compounds. Toxicity of ivermectin in some collie dogs may be explained by a 4-bp deletion mutation of the mdr1 gene resulting in a frame shift, generating stop codons that prematurely terminate synthesis of P-glycoprotein. Additionally, sub-populations of CF-1 identified as expressing reduced levels of P-glycoprotein exhibit increased toxicity to substrates of this transporter. Furthermore, while the traditional view of drug-drug interactions is alteration in drug clearance mediated through a change in hepatic drug metabolism, some of these changes may arise through competition for binding sites on P-glycoprotein in the blood-brain barrier, resulting in reduced extracellular efflux and enhanced CNS toxicity. In conclusion, P-glycoprotein is an integral component of the human blood brain barrier and plays a central role in limiting drug uptake into the brain. Altered expression or function of p-glycoprotein could conceivably allow elevation of brain concentrations of ivermectin and produce severe neurotoxicity. This might arise through a genetic polymorphism in p-glycoprotein or co-administration of ivermectin with a drug or foodstuff that might inhibit this efflux transporter.
Although clinical and veterinary usage of the broad-spectrum anthelmintic agent ivermectin has generally been free of serious adverse events since 1991, a number of clinical cases with and without neurological manifestations, including coma lasting 2–3 days, have been reported after ivermectin treatment of individuals infected with Onchocerca volvulus who also had concomitant Loa loa infection with a high levels of microfilariae, typically > 30,000 microfilariae /ml of blood [1, 2]. Although a cause and effect relationship has yet to be established, it is advised that individuals who warrant treatment with ivermectin for any reason and have had significant exposure to Loa loa-endemic areas of West and Central Africa, pre-treatment assessment for loaiasis and careful post-treatment follow-up should be implemented. The aim of this brief review is to assess whether any of these events could be attributed to the pharmacology of ivermectin, notably its affinity for the P-glycoprotein drug transporter, which might serve to reduce oral bioavailability of ivermectin and prevent its uptake into the brain, thereby preventing potentially fatal neurotoxicity.
Chemistry and Pharmacology of the Macrocyclic Lactones
The Blood Brain Barrier and p-glycoprotein
Neurotoxicology of Ivermectin
The principal toxicological effects of ivermectin seen in rats are ataxia, ptosis and decreased activity, and in dogs are mydriasis, tremors, ataxia and anorexia  with enhanced sensitivity in infant rats that have a poorly developed blood brain barrier (with regard to their deficiency in p-glycoprotein), and show higher brain plasma drug concentration ratios than adult rats. During the development of Mectizan® two reports, one from the veterinary use of ivermectin and another from a pre-clinical toxicology study, influenced the clinical development of Mectizan® such that a more conservative approach was taken . First, it was noticed that a sub-population of collie dogs was remarkably sensitive to ivermectin-induced neurotoxicity . This occurs at doses that are 1/200th of the dose required to cause toxicity in other dogs. Neurological manifestations of ivermectin in susceptible dogs include, hypersalivation, ataxia, blindness, coma, respiratory compromise, and death. It is now known that such ivermectin-sensitive collies exhibit a 4-bp deletion mutation of the mdr1 gene that results in a frame shift, generating several stop codons that prematurely terminate synthesis of P-glycoprotein. Dogs that are homozygous for the mutation display the ivermectin sensitive phenotype, whereas those homozygous normal or heterozygous show no increased sensitivity to ivermectin . Second, sub populations of CF-1 mice were identified as expressing reduced levels of P-glycoprotein, including a population that does not produce P-glycoprotein, identified as (-/-) . Animals with the wild type (+/+) or deficient genotypes differ markedly in their sensitivity to ivermectin neurotoxicity and teratology, attributable to differential accumulation of ivermectin in brain and foetus. CF1 p-glycoprotein deficient mice are phenoptypically identical to mdr1a and mdr1b knockout strains established in the laboratory . Using such strains, differences in pharmacokinetics and tissue distribution, particularly accumulation in the brain, of several substances can be demonstrated when compared with p-glycoprotein wild type mice [29, 30]. These authors clearly established the mdr1a transporter as central to limiting the oral bioavailability and brain uptake of these agents. Studies with ivermectin and the p-glycoprotein substrate cyclosporin A in CF-1 mice demonstrated enhanced absorption of ivermectin and cyclosporin A in the (-/-) strain although there were no differences in the intravenous pharmacokinetics of either drug. Hepatic drug metabolising capacity of (-/-) and (+/+) animals was similar. Concentrations of [3H]-ivermectin and [3H]-cyclosporin A were always higher in the brains of (-/-) mice after oral or intravenous administration but liver concentrations were identical to wild-type mice. These findings clearly demonstrate that changes in ivermectin disposition observed in the (-/-) mice arise through a deficiency in p-glycoprotein rather than any alteration in drug metabolism .
Drug-drug Interactions and Neurotoxicity of Ivermectin
Drug-drug interactions of clinical importance can occur when the pharmacology of a drug is altered by co-administration of another agent. The question of drug-drug interactions among anthelmintics is yet to be answered satisfactorily. This is an important issue, since combination chemotherapy is becoming more widespread as resistance to antiparasitic agents increases. For example albendazole and ivermectin are co-administered in programmes to eliminate lymphatic filariasis . Experiments in human liver microsomes and recombinant enzyme systems have shown that ivermectin and albendazole are both substrates of cytochrome P450 (CYP) 3A4, the most widely distributed human P-450, which is expressed both in the liver and small intestine [32, 33]. We have shown that albendazole is converted to albendazole sulphoxide, the major plasma metabolite in man, by microsomes prepared from human intestinal tissue . There is, therefore, the potential for drug-drug interactions at each of these sites through competition for drug metabolising enzymes. The traditional view is that drug-drug interactions are a result of alterations in drug clearance mediated through a change in hepatic drug metabolism. However, it is becoming clear that some of these changes may arise through competition for binding sites on transport proteins. Fluorescent-labelled ivermectin (BODIPY-ivermectin) is extruded out of brain capillaries by a concentrative mechanism and this export process is reduced by substrates of p-glycoprotein without any change in drug uptake . Moreover, transport of BODIPY-ivermectin is restricted to p-glycoprotein, as it is unaffected by specific inhibitors of related transport proteins and it interacts specifically with p-glycoprotein in functional kidney tubules . These findings suggest that other P-glycoprotein substrates could compete with ivermectin at this site, resulting in reduced extracellular efflux and enhanced CNS toxicity. Such compounds include cyclosporin and HIV protease inhibitors [37, 38]. Increased ivermectin concentrations accompanied by enhanced neurotoxicity of ivermectin in mice has already been demonstrated in the presence of cyclosporin . While an interaction between ivermectin and albendazole could be postulated on the basis of these drugs being co substrates of CY3A4, it is not established that there could be an interaction at the level of p-glycoprotein. Although ivermectin is undeniably a substrate for P-glycoprotein, there is conflicting evidence for albendazole. Intestinal metabolism and secretion of albendazole sulphoxide into the intestinal lumen has been demonstrated and p-glycoprotein is reported to be involved in modulating resistance to albendazole in helminths [40, 41]. However, experiments in Caco-2 cells failed to identify albendazole as a p-glycoprotein substrate or an inhibitor of p-glycoprotein mediated transport of digoxin , suggesting that a combination of the two drugs would not produce an adverse drug reaction based upon an interaction with p-glycoprotein. Albendazole sulphoxide or sulphone were not investigated.
P-glycoprotein is an integral component of the human blood brain barrier and plays a central role in limiting drug uptake into the brain. Altered expression or function of p-glycoprotein could conceivably allow elevation of brain concentrations of ivermectin and produce severe neurotoxicity. This might arise through a genetic polymorphism in p-glycoprotein or co-administration of ivermectin with a drug or foodstuff that might inhibit this efflux transporter. There are relatively few examples of clinically important drug-drug interactions at the blood-brain barrier that involve increased drug exposure within the brain. As mentioned previously, alterations in uptake into the brain may not be manifest as a change in systemic pharmacokinetics and may only become apparent when a pharmacodynamic assessment is made . Furthermore, it may not be possible, with therapeutic doses, to achieve sufficiently high concentrations of a co-substrate to allow a drug-drug interaction to take place. For example, a range of antipsychotic, antiemetic and Ca2+ blocking agents failed to produce effective inhibition of p-glycoprotein despite the achievement of suitable concentrations within the systemic circulation . Perhaps more importantly, it should be remembered that clinical and laboratory findings suggest that Mectizan®-associated L. loa encephalopathy is consistent with an embolic process triggered by massive microfilarial death. It may also involve circulating immune complexes or polymorphic inflammatory responses. Moreover, serious or even fatal encephalopathy may develop spontaneously, or following treatment with other microfilaricides such as diethylcarbamazine . It is therefore questionable whether any of the neurological sequelae associated with administration of ivermectin are directly related to the drug and therefore it seems unlikely that any alterations in the pharmacokinetics of ivermectin for whatever reason would result in a more severe adverse reaction.
Geoffrey Edwards was sole author.
Original work referred to by the author (references 32 and 34) was supported by a postgraduate studentship from GlaxoSmithKline Pharmaceuticals to Dr Helen Rawden.
- Gardon J, Gardon-Wendel N, Demanga-Ngangue , Kamgno J, Chippaux JP, Boussinesq M: Serious reactions after mass treatment of onchocerciasis with ivermectin in an area endemic for Loa loa infection. Lancet. 1997, 350: 18-22. 10.1016/S0140-6736(96)11094-1.PubMedView Article
- Boussinesq M, Gardon J, Gardon-Wendel N, Kamgno J, Ngoumou P, Chippaux JP: Three probable cases of Loa loa encephalopathy following ivermectin treatment for onchocerciasis. Am J Trop Med Hyg. 1998, 58: 461-469.PubMed
- Burg RW, Stapley EO: Isolation and characterisation of the producing organism. In: Ivermectin and Ababmectin. Edited by: William C Campbell. 1989, New York, Berlin, Heidelberg, London, Paris, Tokyo, Springer-Verlag, 24-32.View Article
- Fisher MH, Mrozik H: Chemistry. In: Ivermectin and Ababmectin. Edited by: William C Campbell. 1989, New York, Berlin, Heidelberg, London, Paris, Tokyo, Springer-Verlag, 1-23.View Article
- Geary TG, Sims SM, Thomas EM, Vanover L, Davis JP, Winterrowd CA, Klein RD, Ho NF, Thompson DP: Haemonchus contortus: ivermectin-induced paralysis of the pharynx. Exp Parasitol. 1993, 77: 88-96. 10.1006/expr.1993.1064.PubMedView Article
- Martin RJ: An electrophysiological preparation of pharyngeal muscle reveals a glutamate-gated chloride channel sensitive to the avermectin analogue milbemycin D. Parasitology. 1996, 120: 587-594.
- Kotze AC: ffects of macrocyclic lactones on ingestion in susceptible and resistant Haemonchus contortus larvae. J Parasitol. 1998, 84: E631-635. 10.2307/3284740.View Article
- Sangster NC, Gill J: Pharmacology of anthelmintic resistance. Parasitol Today. 1999, 15: 141-146. 10.1016/S0169-4758(99)01413-1.PubMedView Article
- Cully DF, Wilkinson H, Vassilatis DK, Etter A, Arena JP: Molecular biology and electrophysiology of glutamate-gated chloride channels of invertebrates. Parasitology. 1996, 113: 191-200. 10.1017/S0031182000077970.View Article
- Dent JA, Davis MW, Avery L: avr-15 encodes a chloride channel subunit that mediates inhibitory glutaminergic neurotransmission and ivermectin sensitivity in Caenorhabditis elegans. EMBO J. 1997, 16: 5867-5879. 10.1093/emboj/16.19.5867.PubMedPubMed CentralView Article
- Vassilatis DK, Arena JP, Plasterk RHA, Wilkinson HA, Schaeffer JM, Cully DF, Van der Ploeg LHT: Genetic and biochemical evidence for a novel avermectin sensitive chloride channel in Caenorhabditis elegans. J Biol Chem. 1997, 272: 33167-33174. 10.1074/jbc.272.52.33167.PubMedView Article
- Kohler P: The biochemical basis of anthelmintics resistance. Int J Parasitol. 2001, 31: 336-345. 10.1016/S0020-7519(01)00131-X.PubMedView Article
- Etter A, Cully DF, Liu KK, Reiss B, Vassilatis DK, Schaeffer JM, Arena JP: Picrotoxin blockade of invertebrate glutamate gated chloride channels: subunit dependence and evidence for binding within the pore. J Neurochem. 1999, 72: 318-326.PubMed
- Kwei GY, Alvaro RF, Chen Q, Jenkins HJ, Hop CEAC, Keohane CA, Ly VT, Strauss JR, Wang RW, Wang Z, et al: Disposition of ivermectin and cyclosporin A in CF-1 mice deficient in MDR1A p-glycoprotein. Drug Metab Dispos. 1999, 27: 581-587.PubMed
- Tamai I, Atsuji A: Transporter-mediated permeation of drugs across the blood brain barrier. J Pharm Sci. 2000, 89: 1371-1388. 10.1002/1520-6017(200011)89:11<1371::AID-JPS1>3.0.CO;2-D.PubMedView Article
- Ayrton A, Morgan P: Role of transport proteins in drug absorption, distribution and excretion. Xenobiotica. 2001, 31: 469-497. 10.1080/00498250110060969.PubMedView Article
- Cordon-Cardo C, O'Brien JP, Casals D, Rittman Graver L, Beidler JL, Melamed MR, Bertino JR: Multidrug resistance gene P-glycoprotein is expressed by endothelial cells at blood-brain barrier sites. Proc Natl Acad Sci USA. 1989, 86: 695-698. 10.1073/pnas.86.2.695.PubMedPubMed CentralView Article
- Huai-Yun H, Secrest DT, Mark KS, Carney D, Brandquist C, Elmquist WF, Miller DW: Expression of multidrug resistance-assocaited protein (MRP) in brain microvessel endothelial cells. Biochem Biophys Res Comm. 1998, 243: 816-820. 10.1006/bbrc.1997.8132.PubMedView Article
- Kusuhara H, Suzuki H, Naito M, Tsuro T, Sugiyama Y: Characterisation of efflux transport of organic anions in a mouse brain capillary endothelial cell line. J Pharm Exp Ther. 1998, 285: 1260-1265.
- Gao B, Steiger B, Noe B, Fritschy JM, Meier PJ: Localisation of the organic anion transporting polypeptide 2 (Oatp 2) in capillary endothelium and choroids plexus epithelium of rat brain. J Histochem Cytochem. 1999, 47: 1255-1263. 10.1177/002215549904701005.PubMedView Article
- Hoffmeyer S, Burk O, von Richter O, Arnold HP, Brockmoller J, Johne A, Cascorbi L, Gerlo T, Roots I, Eichelbaum M, Brinkmann U: Functional polymorphisms of the human multidrug-resistance gene: multiple sequence variations and correlation of one allele with P-glycoprotein expression and activity in vivo. Proc Natl Acad Sci USA. 2000, 97: 3473-3478. 10.1073/pnas.050585397.PubMedPubMed CentralView Article
- Kim RB, Leake B, Choo E, Dresser GK, Kubra SV, Schwarz UL, Taylor A, Xie HG, Stein CM, Wood AJJ, et al: Identification of functionally important MDR1 variant alleles among African-American and Caucasian subjects. Drug Metab Rev. 2000, 32 (suppl 2): 199-
- Lankas GR, Gordon LR: Toxicology. In: Ivermectin and Ababmectin. Edited by: William C Campbell. 1989, New York, Berlin, Heidelberg, London, Paris, Tokyo, Springer-Verlag, 89-112.View Article
- Brown KR: Changes in the use profile of Mectizan®: 1987–1997. Ann Trop Med Parasitol. 1998, 92: S61-S64. 10.1080/00034989859564.PubMedView Article
- Paul AJ, Tranquilli WJ, Seward RL, Todd KS, DiPietro JA: Clinical observations in collies given ivermectin orally. Am J Vet Res. 1987, 48: 684-685.PubMed
- Mealey KL, Bentjen SA, Gay JM, Cantor GH: Ivermectin sensitivity in collies is associated with a deletion mutation of the mdr1 gene. Pharmacogenetics. 2001, 11: 727-733. 10.1097/00008571-200111000-00012.PubMedView Article
- Umbenhauer DR, Lankas GR, Pippert TR, Wise LD, Cartwright ME, Hall SJ, Beare CM: Identification of a P-glycoprotein-deficient subpopulation in the CF-1 mouse strain using a restriction fragment length polymorphism. Toxicol Applied Pharmacol. 1997, 146: 88-94. 10.1006/taap.1997.8225.View Article
- Schinkel AH, Smit JJ, van Tellingen O, Beijen JH, Wagenaar E, van Deemter L, Mol CAAM, van der Valk MA, Robanus Maandag EC, Borst F: Disruption of the mouse mdr1a P-glycoprotein gene leads to a deficiency in the blood-brain barrier and to increased sensitivity to drugs. Cell. 1994, 77: 491-502. 10.1016/0092-8674(94)90212-7.PubMedView Article
- Schinkel AH, Wagenaar E, van Deemter L, Mol CAAM, Borst F: Absence of the mdr1a P-glycoprotein in mice affects tissue distribution and pharmacokinetics of dexamethasone, digoxin and cyclosporin A. J Clin Invest. 1995, 96: 1698-1705. 10.1172/JCI118214.PubMedPubMed CentralView Article
- Sparreboom A, van Asperen J, Mayer U, Schinkel AH, Smit JW, Meier DKF, Borst P, Nooijen WJ, Beinen JH, van Tellingen O: Limited oral bioavailability and active epithelial secretion of paclitaxel (taxol) caused by P-glycoprotein in the intestine. Proc Natl Acad Sci USA. 1997, 94: 2013-2035. 10.1073/pnas.94.5.2031.View Article
- Horton J, Witt C, Ottesen EA, Lazdins JK, Addiss DG, Awadzi K, Beach MJ, Belizario VY, Dunyo SK, Espinel M, et al: An analysis of the safety of the single dose, two drug regimens used in programmes to eliminate lymphatic filariasis. Parasitology. 2000, 121: Suppl: S147-160. 10.1017/S0031182000007423.View Article
- Rawden HC, Kokwaro GO, Ward SA, Edwards G: Relative contribution of cytochromes P-450 and flavin-containing monoxygenases to the metabolism of albendazole by human liver microsomes. Br J Clin Pharmacol. 2000, 49: 313-322. 10.1046/j.1365-2125.2000.00170.x.PubMedPubMed CentralView Article
- Zeng , Andrew NW, Arison BH, Luffer-Atlas D, Wang RW: Identification of P4503A4 as the major enzyme responsible for the metabolism of ivermectin by human liver microsomes. Xenobiotica. 1998, 28: 313-321. 10.1080/004982598239597.PubMedView Article
- Rawden HC: Ph.D. thesis. "An investigation of potential chemotherapeutic drug combinations with Albendazole for the treatment of Echinococcus infection". The University of Liverpool. 1999
- Nobmann S, Bauer B, Fricker G: Ivermectin excretion by isolated functionally intact brain endothelial capillaries. Br J Pharmacol. 2001, 132: 722-728. 10.1038/sj.bjp.0703762.PubMedPubMed CentralView Article
- Fricker G, Gutmann H, Droulle A, Drewe J, Miller DS: Epithelial transport of anthelmintic ivermectin in a novel model of isolated proximal kidney tubules. Pharm Res. 1999, 16: 1570-1575. 10.1023/A:1018956621376.PubMedView Article
- Drewe J, Gutmann H, Fricker G, Torok M, Beglinger C, Huwyler J: HIV protease inhibitor ritonavir: a more potent inhibitor of P-glycoprotein than the cyclosporine analog SDZ PSC 833. Biochem Pharmacol. 1999, 15: 1147-1152. 10.1016/S0006-2952(99)00026-X.View Article
- Gutmann H, Fricker G, Drewe J, Toeroek M, Miller DS: Interactions of HIV protease inhibitors with ATP-dependent drug export proteins. Mol Pharm. 1999, 56: 383-389.
- Marques-Santos LF, Bernardo RR, de Paula EF, Rumjanek VM: Cyclosporin A and trifluperazine, two resistance modulating agents, increase ivermectin neurotoxicity in mice. Pharmacol Toxicol. 1999, 84: 125-129. 10.1111/j.1600-0773.1999.tb00887.x.PubMedView Article
- Nare B, Liu Z, Prichard RK, Georges EE: Benzimidazoles, potent anti-mitotic drugs: substrates for the P-glycoprotein transporter in multidrug-resistant cells. Biochem Pharmacol. 1994, 48: 2215-2222. 10.1016/0006-2952(94)00427-7.PubMedView Article
- Redondo PA, Alvarez AI, Garcia JL, Larrode OM, Merino G, Prieto JG: Presystemic metabolism of albendazole: experimental evidence of an efflux process of albendazole sulphoxide to intestinal lumen. Drug Metab Dispos. 1999, 27: 736-740.PubMed
- Merino G, Alvarez AJ, Prieto JG, Kim RB: The anthelmintic agent albendazole does not interact with p-glycoprotein. Drug Metab Dispos. 2002, 30: 365-369. 10.1124/dmd.30.4.365.PubMedView Article
- Ibrahim S, Peggins J, Knapton A, Licht T, Aszalos A: Influence of antipsychotic, antiemetic and Ca2+ channel blocker drugs on the cellular accumulation of the anticancer drug daunorubicin: P-glycoprotein modulation. J Pharm Exp Ther. 2000, 295: 1276-1283.
- Carme B, Boulesteix J, Boutes H, Puruehnce MF: Five cases of encephalitis during treatment of loaiasis with diethylcarbamazine. Am J Trop Med Hyg. 1991, 44: 684-690.PubMed
This article is published under license to BioMed Central Ltd. This article is in the Public Domain