After administration, anthelmintics are usually absorbed into the bloodstream and transported to different parts of the body, including the liver, where they may be metabolized and eventually excreted in the feces and urine. The disposition of anthelmintics throughout the body is considerably more complex than can be described by a set of pharmacokinetic parameters in the peripheral circulation. Improved drug performance requires knowledge of drug behavior in the multicompartmental system, including the complex interaction between formulation and route of administration, physicochemical properties of the compound, and physiology of the compartment into which the drug is distributed.

While many helminth parasites reside in the lumen or close to the mucosa, others live at sites such as the liver and lungs; for action against these, absorption of drug from the GI tract, injection site, or skin is essential. Intestinal parasites come in contact not only with the unabsorbed drug passing through the GI tract but also with the absorbed fraction in the blood as they feed on the intestinal mucosa, and with any that is recycled into the gut. This is an important aspect of efficacy of many of the benzimidazoles.

The pharmacokinetics of an anthelmintic, its rate of metabolism and excretion, and its safety profile determines the length of the withdrawal time; this period can vary among species and can also be affected by the route of administration and the dose. The usual site of metabolism of anthelmintics is the liver, where oxidation and cleavage reactions commonly occur.

Benzimidazoles and Probenzimidazoles

With a few exceptions, eg, albendazole, oxfendazole, and triclabendazole, only limited amounts of any of the benzimidazoles are absorbed from the GI tract of the host. The limited absorption is probably related to the poor water solubility of these drugs. The little absorption that occurs is generally rapid, 2–7 hr after dosing with flubendazole and 6–30 hr after dosing with albendazole, fenbendazole, and oxfendazole, depending on the species. Many of the benzimidazoles and their metabolites re-enter the GI tract by passive diffusion, but the biliary route is the most important pathway for secretion and recycling of benzimidazoles to the GI tract.

A number of benzimidazoles (eg, febantel, thiophanate, netobimin) exist in the form of prodrugs that must be metabolized in the body to the biologically active benzimidazole carbamate nucleus. Febantel is hydrolyzed to the active metabolite fenbendazole, and netobimin undergoes processes of reduction, cyclization, and oxidation to yield albendazole sulfoxide. Benzimidazole sulfoxides such as oxfendazole and albendazole sulfoxide bind poorly to parasite β-tubulin and probably act as prodrugs for fenbendazole and albendazole, respectively. The thiometabolites have high affinity for helminth tubulin.

Metabolism of the benzimidazoles is variable and may alter their activity; eg, albendazole is rapidly and reversibly oxidized to its sulfoxide form. The sulfoxide may be irreversibly oxidized to its sulfone, which is significantly less active than the sulfoxide. Similarly, fenbendazole and oxfendazole (fenbendazole sulfoxide) are interchangeable, but the oxidation product fenbendazole sulfone is less active and is not reduced back to the sulfoxide or thio metabolites.

In ruminants, the benzimidazoles are most effective if deposited into the rumen. Administration directly into the abomasum, via the esophageal groove, may shorten the duration for drug absorption and increase the rate of excretion in the feces, which may reduce efficacy. For example, immediate arrival of oxfendazole in the abomasum after dosing reduces its efficacy from 91% to 45% against thiabendazole-resistant strains of Haemonchus contortus. The rumen acts as a drug reservoir from which plasma concentrations can be sustained, slowing the passage of unabsorbed drug through the GI tract.

Imidazothiazoles

The absorption and excretion of levamisole is rapid and not affected by the route of administration or ruminal bypass because it is highly soluble. In cattle, blood levels of levamisole peak <1 hr after SC administration. These concentrations decline rapidly; 90% of the total dose is excreted in 24 hr, largely in the urine.

Tetrahydropyrimidines

Pyrantel tartrate (or citrate) is well absorbed by pigs and dogs, less well by ruminants. The pamoate salt (synonym embonate) of pyrantel is poorly soluble in water; this offers the advantage of reduced absorption from the gut and allows the drug to reach and be effective against parasites in the large intestine, which makes it useful in horses and dogs. Metabolism of pyrantel is rapid, and the metabolites are excreted rapidly in the urine (40% of the dose in dogs); some unchanged drug is excreted in the feces (principally in ruminants). Blood levels usually peak 4–6 hr after PO administration.

Morantel is the methyl ester analog of pyrantel and, in ruminants, it tends to be safer and more effective than pyrantel. It is absorbed rapidly from the upper small intestine of sheep and metabolized rapidly in the liver; ~17% of the initial dose is excreted in the urine as metabolites within 96 hr after dosing.

Macrocyclic Lactones

Macrocyclic lactones are hydrophobic, an important characteristic of this class of anthelmintics. Regardless of their route of administration, macrocyclic lactones are distributed throughout the body and some concentrate in adipose tissue. Liver tissue contains the highest residue for the longest, reflecting the route of elimination. While the magnitude of lipophilicity differs among chemical types, the limited vascularization and slow turnover rate of body fat and the slow rate of release or exchange of drug from these lipid reserves can prolong the residence of drug in the peripheral plasma. Ivermectin is arguably the least lipophilic macrocyclic lactone, with the possible exception of eprinomectin. Moxidectin is ~100 times more lipophilic than ivermectin. Doramectin is less lipophilic than moxidectin, but more than ivermectin or eprinomectin.

Ivermectin was the first commercially available macrocyclic lactone and has been the most extensively studied. When given IV, ivermectin has an elimination half-life of 32–178 hr, depending on species. Despite the higher dose rate of the injectable formulation in pigs (300 μg/kg) compared with cattle (200 μg/kg), the maximum concentration (C_max) and area under curve (AUC) in peripheral plasma in pigs are about one-third those in cattle. While the elimination half-life after SC and IV administration of ivermectin is of similar duration, slow absorption from the injection site may broaden the concentration-time profile, with C_max in peripheral plasma of cattle occurring as late as 96 hr. The C_max and AUC in pigs and goats are considerably lower than those in cattle, horses, and sheep. Pigs, and possibly goats, may metabolize ivermectin faster than other species.

In ruminants, the macrocyclic lactones are, like benzimidazoles, most effective if deposited directly into the rumen. A 3- to 4-fold decrease in C_max and AUC of ivermectin after intra-abomasal compared with intraruminal administration has been reported. Significantly, time to maximal concentration of ivermectin was reduced from 23 hr to 4 hr with the former route of delivery.

Concentrations of ivermectin are high in digesta sampled from the distal intestine, indicating that biliary secretion is an important pathway for clearance of macrocyclic lactones. This pathway also has been conclusively demonstrated for clearance of benzimidazole compounds. The extended high concentration in bile is influenced by prolonged exchange of drug from lipid reserves and the enterohepatic recycling of biliary compounds through the portal and biliary pools. The macrocyclic lactones are primarily excreted in the feces, the remainder (<10%) in the urine. The more lipophilic macrocyclic lactones are also excreted in milk.

Salicylanilides and Substituted Phenols

Secretion via the liver and bile is especially important for drugs active against adult Fasciola spp. The fasciolicidal effects of salicylanilides (such as rafoxanide) in sheep depend on persistence of the drug in plasma, which influences their transport throughout the body and rate of elimination. Closantel, rafoxanide, and oxyclozanide have long terminal half-lives in sheep (14.5, 16.6, and 6.4 days, respectively), which are related to the high plasma-protein binding (>99%) of these 3 drugs. Residues in liver are detectable for weeks after administration. Associated with persistence, however, is the need for longer withholding periods. Oxyclozanide also is bound to plasma protein and then metabolized in the liver to the anthelmintically active glucuronide and excreted in high concentration in the bile duct, where it encounters the mature flukes.

Immature flukes in the liver parenchyma ingest mainly liver cells, which contain little anthelmintic; plasma-protein binding limits entry of the drug into the tissue cells. As the flukes grow and migrate through the liver, they cause extensive hemorrhaging and come into contact with anthelmintic bound to plasma protein. When they reach the bile ducts, they are in the main excretory channels for the active metabolites of the fasciolicides and are exposed to toxic concentrations. This may explain why mature flukes are more vulnerable to most fasciolicides than immature ones. The higher concentrations of fasciolicides and their metabolites in feces than in urine suggest that the bile ducts are their main excretory pathways. Diamfenetide is metabolized in the gut, and to a greater extent in the liver, to an active metabolite that can enter hepatic cells and exert its antiparasitic effect against very young stages of the fluke.

It is important to understand the pharmacokinetics of prescribed anthelmintics. For example, nitroxynil has good efficacy against Fasciola hepatica in cattle and sheep and against Haemonchus contortus, but because rumen bacteria metabolize and destroy the activity of nitroxynil, it must be injected.

Last full review/revision March 2012 by Edwin Claerebout, DVM, PhD, DEVPC; Jozef Vercruysse, DVM, DEVPC