Arprinocid, an inhibitor of hypoxanthine-guanine transport

Integrating genetics and genomics to identify new leads for the control ofEimeriaspp

Eimerian parasites display a biologically interesting range of phenotypic variation. In addition to a wide spectrum of drug-resistance phenotypes that are expressed similarly by many other parasites, theEimeriaspp. present some unique phenotypes. For example, unique lines ofEimeriaspp. include those selected for growth in the chorioallantoic membrane of the embryonating hens egg or for faster growth (precocious development) in the mature host. The many laboratory-derived egg-adapted or precocious lines also share a phenotype of a marked attenuation of virulence, the basis of which is different as a consequence of thein ovoorin vivoselection procedures used. Of current interest is the fact that some wild-type populations ofEimeria maximaare characterized by an ability to induce protective immunity that is strain-specific. The molecular basis of phenotypes that defineEimeriaspp. is now increasingly amenable to investigation, both through technical improvements in genetic linkage studies and the availability of a comprehensive genome sequence for the caecal parasiteE. tenella. The most exciting phenotype in the context of vaccination and the development of new vaccines is the trait of strain-specific immunity associated withE. maxima. Recent work in this laboratory has shown that infection of two inbred lines of White Leghorn chickens with the W strain ofE. maximaleads to complete protection to challenge with the homologous parasite, but to complete escape of the heterologous H strain, i.e. the W strain induces an exquisitely strain-specific protective immune response with respect to the H strain. This dichotomy of survival in the face of immune-mediated killing has been examined further and, notably, mating between a drug-resistant W strain and a drug-sensitive H strain leads to recombination between the genetic loci responsible for the specificity of protective immunity and resistance to the anticoccidial drug robenidine. Such a finding opens the way forward for genetic mapping of the loci responsible for the induction of protective immunity and integration with the genome sequencing efforts.

A genetic linkage map of the apicomplexan protozoan parasite Eimeria tenella

Apicomplexan protozoan parasites have complex life cycles that involve phases of asexual and sexual reproduction. Some genera have intermediate insect hosts, for example, Plasmodium spp. (the cause of malaria), but related genera such as Eimeria spp. (causative agents of coccidiosis in poultry) have a direct life cycle occurring in only a single host. Mechanisms that regulate the life cycles of apicomplexan parasites are unknown, but the intracellular growth of avian Eimeria spp. is easily shortened by serial selection for the first parasites to complete the transition from asexual to sexual reproduction (to yield so-called precocious lines). To investigate the genetic basis of such an abbreviated life cycle, we have used the species E. tenella and analyzed the inheritance of 443 polymorphic DNA markers in 22 recombinant cloned progeny derived from a cross between parents that had selectable phenotypes of precocious development or resistance to an anticoccidial drug. The markers were placed in 16 linkage groups (which defined 12 chromosomes) and a further 57 unlinked groups. Two linkage groups showed an association (P =.0105) with the traits of precocious development or drug-resistance and were mapped to chromosome 2 (ca 1.2 Mbp) and chromosome 1 (ca 1.0 Mbp), respectively. The map provides a framework for further studies on the identification of genetic loci implicated in the regulation of the life cycle of an important protozoan parasite and a representative of a major taxonomic group. [A table with the segregation data is available as an online supplement at http://www.genome.org.]

Purine base transport in wild-type and mycophenolic acid-resistant Tritrichomonas foetus

The purine base transport systems of wild-type and mycophenolic acid-resistant (MPAR) Tritrichomonas foetus have been characterized. Wild-type T. foetus has two carriers, one for hypoxanthine (Km = 0.7 +/- 0.3 mM, Vm = 80 +/- 20 pmol microliters-1min-1) and guanine (Km = 0.09 +/- 0.02 mM, Vm = 17 +/- 3 pmol microliters-1min-1), and a second for xanthine (Km = 0.6 +/- 0.2 mM, Vm = 25 +/- 5 pmol microliters-1min-1). Adenine transport was not saturable (k = 0.16 +/- 0.01 min-1) and therefore appears to enter the parasite by passive diffusion through the membrane. T. foetus MPAR has lost the hypoxanthine/guanine transporter. Xanthine and adenine transport are similar in wild-type and MPAR T. foetus. No purine nucleoside transporter could be identified.

Toxoplasma gondii: in vivo and in vitro studies of a mutant resistant to arprinocid-N-oxide

The anticoccidial drug arprinocid and arprinocid-N-oxide, a metabolite produced in vivo, blocked the growth of Toxoplasma gondii in human fibroblasts. The more potent arprinocid-N-oxide inhibited growth by 50% at 20 ng/ml while arprinocid inhibited at 2 micrograms/ml. For both drugs, the host cell was less sensitive than was the parasite. Hypoxanthine did not reverse the antitoxoplasma activity of either drug. We isolated a parasite mutant, R-AnoR-1 that was 16- to 20-fold more resistant to arprinocid-N-oxide than was the wild type RH T. gondii. This mutant was not resistant to arprinocid in vitro. Arprinocid in a daily oral dose of 136 micrograms regularly protected mice against an otherwise fatal infection with RH T. gondii and 55 micrograms had some protective effect. However, all mice infected with R-AnoR-1 and treated with 360 micrograms arprinocid per day died. Since this mutant is fully sensitive to arprinocid, the form of the drug that is therapeutically active in vivo cannot be arprinocid and is likely to be arprinocid-N-oxide.

The interactions between drugs and the parasite surface

The interrelationships between drugs and parasite surfaces are considered under the headings of (a) effects on membrane transport, (b) drug uptake mechanisms and (c) effects on surface morphology and function: praziquantel is discussed under a separate heading. The range of chemotherapeutic compounds that cause permeability changes and concomitant morphological disruption is discussed in terms of mode of drug action. Interpretation of the available data renders it difficult to identify the primary mode of action in the drugs considered. Drug uptake mechanisms are known for relatively few compounds; drug resistance as a function of drug acquisition is discussed. The role of the parasite surface as a specific drug target is argued.

Anticoccidial activity of 9-(2-chloro-6-fluorobenzyl)-6-methylaminopurine and 9-(2-chloro-6-fluorobenzyl)-6-dimethylaminopurine against Eimeria tenella, E. acervulina, and E. maxima in battery trials

In a series of battery experiments utilizing 9-day-old White Leghorn male chicks, 9-(2-chloro-6-fluorobenzyl)-6-methylaminopurine (VM 6387) and 9-(2-chloro-6-fluorobenzyl)-6-dimethylaminopurine (VM 6736) showed remarkable anticoccidial activity against Eimeria tenella, E. acervulina, and E. maxima. The effective dose range was estimated from the results of the efficacy test against E. tenella on the basis of improvement of body weight gain and clinical signs of infection. The tests included dietary levels of 60-100 ppm of VM 6387 and 70-110 ppm of VM 6736. Both compounds gave good protection against E. acervulina and E. maxima at the same dose range. The sporulation of oocysts obtained from cecal contents of birds fed lower levels of VM 6387 was inhibited. Studies of the effects of VM 6387 on stages of the E. tenella life cycle demonstrated that the compound possesses remarkable activity at 1-5 days post-infection corresponding to the period of schizogony, as well as prolonged activity thereafter, when the parasite was undergoing gametogenesis.

Morphological effects of arprinocid on developmental stages of Eimeria tenella and E. brunetti

Medication of chicks with 70 p.p.m. arprinocid in the food, starting 2 days prior to inoculation with Eimeria tenella, resulted in decreased oocyst production and oocyst sporulation. The main morphological alteration seen during the development of the second-generation schizont was a reduction in the number of second-generation merozoites due to incomplete merogony, resulting in large masses of vacuolated residual cytoplasm. In places around the developing macrogamete the integrity of the parasitophorous vacuole was lost by either the host cell limiting membrane making contact with the parasite pellicle, or the disruption of one or both of these surfaces apparently brought about by the intervention of the host cell mitochondria. Thirty p.p.m. arprinocid reduced sporulation of the oocysts of E. brunetti and caused enlargement of the perinuclear space in the late merozoites and swelling of the endoplasmic reticulum around wall-forming bodies II.

The chemotherapy of protozoal infections of veterinary importance

The use of drugs in domestic animals is dominated by considerations of cost-effectiveness and profitability. They are extensively used against coccidial infections of poultry where they are an important factor in intensive husbandry. Ionophore antibiotics, which dominate this field, may have applications in ruminants. Imidocarb is of therapeutic and prophylactic value against babesial infections anf there are new prospects for control of theileriasis. Effective drugs for the control of African trypanosomiasis are limited and attention is being given to alternative uses of available compounds.

A comparative study of the biological activities of arprinocid and arprinocid-1-N-oxide

The livers of chickens maintained on a diet containing an effective in vivo level (70 ppm) of arprinocid, an anticoccidial agent, were found to contain 0.64 ppm of this drug and 0.33 ppm of a metabolite, arprinocid-1-N-oxide. When arprinocid was tested against Eimeria tenella in cultures of chick kidney epithelial cells, which do not metabolize the drug appreciably, it was found to block the development of the parasite with an ID50 of 20 ppm. In the same test system, the 1-N-oxide blocks coccidial development with an ID50 of 0.30 ppm. These observations strongly suggest that it is the 1-N-oxide and not arprinocid itself which is the active anticoccidial moeity when arprinocid is fed to the chickens. The action of arprinocid in kidney epithelial cells is partially reversed by excess hypoxanthine, an observation consistent with inhibition of transmembrane transport of purines as the in vitro mechanism of action of this compound. On the other hand, the in vitro anticoccidial action of the 1-N-oxide is not reversed by excess hypoxanthine. Moreover, the 1-N-oxide is less effetive than arprinocid as a purine transport inhibitor. It follows that the anticoccidial action of arprinocid-1-N-oxide is not principally a consequence of inhibition of purine transport.