Influence of verapamil on the efflux and metabolism of 14C moxidectin in cultured rat hepatocytes

Moxidectin inhibits glioma cell viability by inducing G0/G1 cell cycle arrest and apoptosis

Moxidectin (MOX), a broad‑spectrum antiparasitic agent, belongs to the milbemycin family and is similar to avermectins in terms of its chemical structure. Previous research has revealed that milbemycins, including MOX, may potentially function as effective multidrug resistance agents. In the present study, the impact of MOX on the viability of glioma cells was examined by MTT and colony formation assay, and the molecular mechanisms underlying MOX‑mediated glioma cell apoptosis were explored by using flow cytometry and apoptosis rates. The results demonstrated that MOX exerts an inhibitory effect on glioma cell viability and colony formations in vitro and xenograft growth in vivo and is not active against normal cells. Additionally, as shown by western blot assay, it was demonstrated that MOX arrests the cell cycle at the G0/G1 phase by downregulating the expression levels of cyclin‑dependent kinase (CDK)2, CDK4, CDK6, cyclin D1 and cyclin E. Furthermore, it was revealed that MOX is able to induce cell apoptosis by increasing the Bcl‑2‑associated X protein/B‑cell lymphoma 2 ratio and activating the caspase‑3/‑9 cascade. In conclusion, these results suggest that MOX may inhibit the viability of glioma cells by inducing cell apoptosis and cell cycle arrest, and may be able to function as a potent and promising agent in the treatment of glioma.

Treatment of MDR1 mutant dogs with macrocyclic lactones

P-glycoprotein, encoded by the multidrug resistance gene MDR1, is an ATP-driven drug efflux pump which is highly expressed at the blood-brain barrier of vertebrates. Drug efflux of macrocyclic lactones by P-glycoprotein is highly relevant for the therapeutic safety of macrocyclic lactones, as thereby GABA-gated chloride channels, which are confined to the central nervous system in vertebrates, are protected from high drug concentrations that otherwise would induce neurological toxicity. A 4-bp deletion mutation exists in the MDR1 gene of many dog breeds such as the Collie and the Australian Shepherd, which results in the expression of a non-functional P-glycoprotein and is associated with multiple drug sensitivity. Accordingly, dogs with homozygous MDR1 mutation are in general prone to neurotoxicity by macrocyclic lactones due to their increased brain penetration. Nevertheless, treatment of these dogs with macrocyclic lactones does not inevitably result in neurological symptoms, since, the safety of treatment highly depends on the treatment indication, dosage, route of application, and the individual compound used as outlined in this review. Whereas all available macrocyclic lactones can safely be administered to MDR1 mutant dogs at doses usually used for heartworm prevention, these dogs will experience neurological toxicity following a high dose regimen which is common for mange treatment in dogs. Here, we review and discuss the neurotoxicological potential of different macrocyclic lactones as well as their treatment options in MDR1 mutant dogs.

In vitro and in vivo interaction of moxidectin with BCRP/ABCG2

The study characterizes the interaction between BCRP/ABCG2 and moxidectin by means of cellular transport, and pharmacokinetic studies in Bcrp1 (-/-) and wild-type mice. Milbemycin moxidectin ([(3)H]-moxidectin) was tested for its ability to be transported across MCDK-II epithelial monolayer cultures transfected with BCRP. In a second approach, accumulation assays by BCRP-expressing Xenopus laevis oocytes were carried out. Finally, pharmacokinetic studies were performed in order to establish the role of the transporter in milk secretion and tissue distribution. The efflux was negligible in polarized cells but moxidectin was efficiently transported in BCRP-expressing X. laevis oocytes. The transport was blocked by an acridone derivative, a novel BCRP inhibitor. Moxidectin secretion into breast milk was decreased in Bcrp1-knockout mice and the milk to plasma ratio was 2-fold higher in wild-type mice after i.v. administration. Drug accumulation in intestinal content, bile, and intestine was higher in wild-type mice but the plasma concentration was not different. Moxidectin is identified as a BCRP substrate since its Bcrp1-mediated secretion into breast milk and the involvement of Bcrp1 in intestinal and bile secretion has been demonstrated. This interaction has pharmacokinetic and toxicological consequences. The most important toxicological consequences of the interaction between BCRP and moxidectin may be related with the presence of drug residues in milk.

ABC transporter modulation: a strategy to enhance the activity of macrocyclic lactone anthelmintics

The emergence of parasites resistant to anthelmintic macrocyclic lactones (MLs) threatens to severely limit current parasite control strategies. Improving the current ML-based chemotherapy to perpetuate the efficacy of this broad-spectrum class of anthelmintics would be advantageous. In recent years it has become evident that the absorption, distribution and elimination of the MLs in hosts and parasites are under the control of multidrug resistance transporters (MDRs) such as P-glycoproteins. Theoretically, the inhibition of these transporters should result in an increase of the drug concentration in the organisms and higher treatment efficiency. This opinion article will discuss the recent findings in this research field and assess the possibilities of this approach being used in the field.

Interaction of macrocyclic lactones with P-glycoprotein: Structure–affinity relationship

P-glycoprotein (P-gp) is involved in the ATP-dependant cellular efflux of a large number of drugs including ivermectin, a macrocyclic lactone (ML) endectocide, widely used in livestock and human antiparasitic therapy. The interactions of P-gp with ivermectin and other MLs were studied. In a first approach, the ability of ivermectin (IVM), eprinomectin (EPR), abamectin (ABA), doramectin (DOR), selamectin (SEL), or moxidectin (MOX) to inhibit the rhodamine123 efflux was measured in recombinant cells overexpressing P-gp. Then, the influence of these compounds on the P-gp ATPase activity was tested on membrane vesicles prepared from fibroblasts overexpressing P-gp. All the MLs tested increased the intracellular rhodamine123. However, the potency of MOX to inhibit P-gp function was 10 times lower than the other MLs. They all inhibited the basal and decreased the verapamil-stimulated P-gp ATPase activity. But SEL and MOX were less potent than the other MLs when competing with verapamil. According to the structural specificity of SEL and MOX, we conclude that the integrity of the sugar moiety is determinant to achieve the optimal interaction of macrocyclic lactones with P-gp. The structure-affinity relationship for interaction with P-gp is important information for improving ML bioavailability and reversal of multidrug resistance (MDR).

Fumagillin, a new P-glycoprotein-interfering agent able to modulate moxidectin efflux in rat hepatocytes

We have tested the ability of two compounds licensed in veterinary medicine: fumagillin and diminazene diaceturate to increase intracellular moxidectin quantity in rat hepatocytes. These compounds significantly increased the quantity of 14C-moxidectin (expressed as area under the time curve concentrations) in cultured rat hepatocytes by 44% and 65% for diminazene and fumagillin treatments respectively. In addition, we have tested these drugs for their interference with P-glycoprotein (P-gp) function in porcine kidney epithelial cells transfected with murine mdr1a (Mdr1a-LLCPK1). We examined the intracellular accumulation of rhodamine 123 (Rho 123) as a functional test to evaluate the effects of these two drugs on P-gp activity. In this model, only fumagillin led to a marked intracellular accumulation of Rho 123. After transforming the data to express the results as a percentage of the accumulation in the presence of the P-gp inhibitor valspodar (VSP), the maximal Rho 123 accumulation was 47% of that with VSP for 100 microm fumagillin. The EC50, the concentration needed to determine 50% of the maximal effect was 34 microm. Fumagillin interacts with P-gp function and appears as a promising compound among registered drugs available, which may optimize the therapeutic use of macrocyclic lactones (MLs).

The interaction between moxidectin and MDR transporters in primary cultures of rat hepatocytes

The interaction of moxidectin (a macrocyclic lactone, ML) with P-glycoprotein (P-gp), multidrug resistance associated proteins (MRPs) and breast cancer resistance protein (BCRP) was studied in primary cultures of rat hepatocytes by measuring the intracellular accumulation of [14C]-moxidectin over 72 h in the presence of specific inhibitors: for P-gp, verapamil (10 microM); for MRPs, MK571 (100 microM), indomethacin (10 microM) and probenecid (3.8 mM); and for BCRP, fumitremorgin C (5 microM). The P-gp and MRP inhibitors increased significantly (P < 0.01) by 48.7%, 49.8%, 49.9% and 57.2% the area under the time-intracellular concentration curve (AUC) of moxidectin in rat hepatocytes, while the BCRP inhibitor, fumitremorgin C, had no effect on the AUC compared with the control. In addition, the mRNAs of all the drug transporters studied were detected in rat hepatocytes from 0 to 72 h. Using this cellular model it has been shown that MRP inhibitors increase moxidectin intracellular concentrations to a similar extent as the P-gp inhibitor. The identification of all the transporters that interact with MLs remains a challenge, which currently concerns several important therapeutic fields.

Interaction of ivermectin with multidrug resistance proteins (MRP1, 2 and 3)

Ivermectin is a potent antiparasitic drug from macrocyclic lactone (ML) family, which interacts with the ABC multidrug transporter P-glycoprotein (Pgp). We studied the interactions of ivermectin with the multidrug resistance proteins (MRPs) by combining cellular and subcellular approaches. The inhibition by ivermectin of substrate transport was measured in A549 cells (calcein or 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, BCECF) and in HL60-MRP1 (calcein). Ivermectin induced calcein and BCECF retention in A549 cells (IC(50) at 1 and 2.5microM, respectively) and inhibited calcein efflux in HL60-MRP1 (IC(50)=3.8microM). The action of ivermectin on the transporters ATPase activity was followed on membranes from Sf9 cells overexpressing human Pgp, MRP1, 2 or 3. Ivermectin inhibited the Pgp, MRP1, 2 and 3 ATPase activities after stimulation by their respective activators. Ivermectin showed a rather good affinity for MRPs, mainly MRP1, in the micromolar range, although it was lower than that for Pgp. The transport of BODIPY-ivermectin was followed in cells overexpressing selectively Pgp or MRP1. In both cell lines, inhibition of the transporter activity induced intracellular retention of BODIPY-ivermectin. Our data revealed the specific interaction of ivermectin with MRP proteins, and its transport by MRP1. Although Pgp has been considered until now as the sole active transporter for this drug, the MRPs should be taken into account for the transport of ivermectin across cell membrane, modulating its disposition in addition to Pgp. This could be of importance for optimizing clinical efficacy of ML-based antiparasitic treatments. This offers fair perspectives for the use of ivermectin or non-toxic derivatives as multidrug resistance-reversing agents.

Selamectin is a potent substrate and inhibitor of human and canine P-glycoprotein

The transport of the antiparasitic agents, ivermectin, selamectin and moxidectin was studied in human intestinal epithelial cell monolayers (Caco-2) and canine peripheral blood lymphocytes (PBL). Both models expressed the mdr1-coded 170 kDa ATP-binding cassette (ABC) transporter P-glycoprotein (P-gp). Fluxes of the P-gp substrate rhodamine-123 (Rh-123) across Caco-2 monolayers showed that ivermectin and selamectin acted as potent P-gp inhibitors with IC50 values of 0.1 microm. In contrast, moxidectin was a weaker P-gp inhibitor with an IC50 of 10 microm. The transport of radiolabelled ivermectin, selamectin and moxidectin through Caco-2 monolayers showed that ivermectin, selamectin and moxidectin were P-gp substrates with secretory/absorptive ratios of 7.5, 4.7 and 2.6 respectively. Secretory transport of [3H]-ivermectin and [3H]-selamectin was blocked by the P-gp inhibitor, verapamil. Ivermectin and selamectin inhibited the efflux of Rh-123 from PBL and the concentration of inhibition was similar to that of verapamil. In contrast, moxidectin did not have a significant effect on Rh-123 efflux from PBL. The data suggest that ivermectin and selamectin are potent P-gp substrates, while moxidectin is a weak one.

Development of a PCR-based diagnostic test detecting a nt230(del4) MDR1 mutation in dogs: verification in a moxidectin-sensitive Australian Shepherd

A subpopulation of dogs of the Collie and Australian Shepherd breeds show increased sensitivity to central nervous actions of ivermectin, doramectin, loperamide, and probably several other drugs. The molecular background for this greater sensitivity is a nonsense mutation in the MDR1 efflux pump, which is part of the functional blood-brain barrier and normally limits drug penetration into the brain. This report describes a rapid PCR-based method for detection of this nt230(del4) MDR1 mutation using a small amount of genomic DNA from blood cells. Thereby, homozygous intact, homozygous mutated, and heterozygous mutated MDR1 genotypes can be clearly differentiated by high resolution polyacrylamide gel electrophoresis. Using this diagnostic test two Collies and one Australian Shepherd were screened for the nt230(del4) MDR1 mutation. The Collies had no history of altered drug sensitivity and showed homozygous intact and heterozygous mutated MDR1 alleles, respectively. However, the Australian Shepherd developed clear signs of neurotoxicity including ataxia, crawling, acoustic and tactile hyperexcitability, and miosis after a single dose of moxidectin (400 microg/kg). For this dog two mutated MDR1 alleles were detected. This report describes for the first time moxidectin neurotoxicosis in a dog with a homozygous MDR1 mutation.

The antiparasitic moxidectin: safety, tolerability, and pharmacokinetics in humans

A study in healthy male volunteers was completed to evaluate the safety, tolerability, and pharmacokinetics of a single oral dose of the antiparasitic moxidectin (MOX). This drug is registered worldwide as a veterinary antiparasitic agent for use in companion and farm animals. This is the first study of MOX in humans. All subjects were between the ages of 18 and 45 years, with normal cardiac, hematologic, hepatic, and renal function. Doses of MOX studied were 3, 9, 18, and 36 mg in cohorts of 6 subjects each (5:1, MOX:placebo). At the 9-mg and 36-mg doses, two separate cohorts were completed, one in the fasted state and one after the consumption of a high-fat breakfast. For all other cohorts, administration was in the fasted state. Safety and tolerability were assessed by physical examinations, ongoing evaluation of adverse events (AEs), and measurement of laboratory values. Pharmacokinetic (PK) samples were collected just prior to dosing and at various time points until 80 days postdose. Safety assessments from all dose groups studied suggested that MOX was generally safe and well tolerated, with a slightly higher incidence of transient, mild, and moderate central nervous system AEs as the dose increased as compared to placebo. The PKs of MOX were dose proportional within the dose range studied, and the elimination half-life (t1/2 elim) was long (mean: 20.2-35.1 days). At the 9-mg and 36-mg doses, a high-fat breakfast was shown to delay and increase the overall absorption but did not increase maximal concentrations when compared to administration in the fasted state. In summary, the results from this study indicate that MOX is safe and well tolerated in humans between the doses of 3 mg and 36 mg.

A role for P-glycoprotein in environmental toxicology

P-Glycoprotein (P-gp) is a transmembrane protein, playing significant roles in the process of drug discovery and development and in pest resistance to pesticides. P-gp affects absorption, disposition, and elimination of different compounds and is mainly expressed in intestines, liver, kidneys, heart, colon, and placenta. The expression of P-gp in the blood-brain barrier (BBB) has been associated with the restricted access of many compounds to the central nervous system. Generated knockout mice by disruption of mdr 1a gene, encoding for P-gp, showed that this protein was expressed in the BBB. The absence or the low levels of P-gp elevated drug concentrations in tissues and decreased drug elimination. P-gp is responsible for resistance of cells to agents, particularly the anticancer drugs, by removing these drugs from cells. Increased expression of P-gp is implicated in decreased HIV drug availability at certain intracellular sites. The role of P-gp in affecting efficacy and toxicity of environmental toxicants such as pesticides and heavy metals has not been adequately investigated. Studies showed that P-gp contributes to resistance to pesticides in certain pest species, and to decrease toxicity by removing compounds from cells in mammals. Placental drug-transporting P-gp plays a significant role in limiting the transport of toxicants such as potential teratogens to the fetus. Several in vitro or in vivo assays, including using P-gp knockout or naturally deficient mice, were described for testing P-gp modulators. The role of P-gp following concurrent exposure to more multiple compounds needs further research. P-gp modulators should be carefully used, since some modulators that reverse P-gp efflux action in vitro may lead to alterations of tissue function and increase toxicity of xenobiotics in normal tissues. Recent reports from the pharmaceutical studies on the significance of P-gp as transporters in altering the efficacy and toxicity clearly highlight the need for further research in interaction with environmental toxicants.

Enhancement of moxidectin bioavailability in lamb by a natural flavonoid: quercetin

Moxidectin is an antiparasitic drug widely used in cattle, sheep and companion animals. Due to the involvement of P-glycoprotein (P-gp) and cytochrome P450 3A in the metabolism of moxidectin, we studied the influence of various P-gp interfering agents (ivermectin, quercetin and ketoconazole) on the metabolism of 14C moxidectin in cultured rat hepatocytes over 72 h. This in vitro study allowed selection of compounds which are able to increase the moxidectin bioavailability in lambs. From this, the modulation of moxidectin pharmacokinetics in plasma of lambs was studied after co-administration of 0.2 mg kg(-1) moxidectin (subcutaneously (SC)) and 0.2 mg kg(-1) ivermectin (SC), or 10 mg kg(-1) quercetin (SC), or 10 mg kg(-1) ketoconazole (orally). Ivermectin and quercetin increased significantly the quantity of 14C moxidectin in the rat hepatocytes. Ketoconazole co-administration led to a higher concentration of moxidectin in the rat hepatocytes. In vivo, only quercetin was able to modify the pharmacokinetics of moxidectin in plasma of lambs by increasing significantly the area under the plasma concentration-time curve. This study allowed the use of a natural agent, quercetin, to improve the bioavailability of moxidectin.