Review article: tolevamer, a novel toxin-binding polymer: overview of preclinical pharmacology and physicochemical properties.

BACKGROUND: Tolevamer is a novel toxin-binding polymer that is currently being investigated in clinical trials for the treatment of patients who have Clostridium difficile-associated diarrhoea. AIMS: To summarize the results of in vitro and in vivo preclinical studies of tolevamer. In contrast to antibiotics, tolevamer binds C. difficile toxins to interrupt toxin-mediated intestinal inflammation and tissue damage, and does not demonstrate direct antimicrobial activity. METHODS: Pharmacokinetics/pharmacodynamics were studied in rats and dogs; efficacy was studied in a hamster model. RESULTS: Studies in rats and dogs indicate that tolevamer is essentially non-absorbed from the gastrointestinal tract and show that drug interactions with commonly used therapies are unlikely. Pharmacologic studies indicate that tolevamer reduces disease severity and recurrence rates in the hamster model of C. difficile-associated diarrhoea and blocks the enterotoxic effects of toxin A in rat ileum. The binding parameters calculated for the interaction of tolevamer with toxins A and B provide a reasonable physicochemical model that supports the potential clinical utility of tolevamer. CONCLUSIONS: These preclinical results are consistent with the effectiveness and safety profile of tolevamer observed in clinical studies in patients with C. difficile-associated diarrhoea.

GT160-246, a toxin binding polymer for treatment of Clostridium difficile colitis.

GT160-246, a high-molecular-weight soluble anionic polymer, was tested in vitro and in vivo for neutralization of Clostridium difficile toxin A and B activities. Five milligrams of GT160-246 per ml neutralized toxin-mediated inhibition of protein synthesis in Vero cells induced by 5 ng of toxin A per ml or 1.25 ng of toxin B per ml. In ligated rat ileal loops, 1 mg of GT160-246 neutralized fluid accumulation caused by 5 microg of toxin A. At doses as high as 80 mg/loop, cholestyramine provided incomplete neutralization of fluid accumulation caused by 5 microg of toxin A. GT160-246 protected 80% of the hamsters from mortality caused by infection with C. difficile, whereas cholestyramine protected only 10% of animals. Treatment of C. difficile-infected hamsters with metronidazole initially protected 100% of the hamsters from mortality, but upon removal of treatment, 80% of the hamsters had relapses and died. In contrast, removal of GT160-246 treatment did not result in disease relapse in the hamsters. GT160-246 showed no antimicrobial activity in tests with a panel of 16 aerobic bacteria and yeast and 22 anaerobic bacteria and did not interfere with the in vitro activities of most antibiotics. GT160-246 offers a novel, nonantimicrobial treatment of C. difficile disease in humans.

Epidemiologic tools for malaria surveillance in an urban setting of low endemicity along the Colombian Pacific coast.

An evaluation of 3 different methods for malaria diagnosis was carried out in an urban area of low endemicity on the Pacific coast of Colombia. Samples were collected from 833 symptomatic patients at a malaria clinic and examined by the polymerase chain reaction (PCR), quantitative buffy coat (QBC; Becton Dickinson, Franklin Lakes, NJ) method, and the traditional thick blood smear. The prevalence of Plasmodium falciparum malaria was 5.88% by thick blood smear, 7.34% by the QBC method, and 21.87% by PCR. The agreement between microscopists was 99.5%. The agreement between the QBC method and thick blood smear was 96.13% (n = 745). Samples positive by PCR but negative by thick blood smear or conversely negative by PCR and positive by thick blood smear were usually of low-level parasitemias. All 3 methods showed agreement in 76.3% of the samples. Sixty-nine (18.8%) samples were positive by PCR but negative by the other 2 methods. Ten samples were positive by both the QBC method and thick blood smear but negative by PCR; most of them had low-level parasitemias. The use of malaria diagnostic methods for epidemiologic surveillance is discussed.

Genetic characterization of an epidemic of Plasmodium falciparum malaria among Yanomami Amerindians.

Malaria parasites are genetically diverse at all levels of endemicity. In contrast, the merozoite surface protein (MSP) alleles in samples from 2 isolated populations of Yanomami Amerindians during an epidemic of Plasmodium falciparum were identical. The nonvariable restriction fragment length polymorphism patterns further suggested that the sequential outbreak comprised only a single P. falciparum genotype. By examination of serial samples from single human infections, the MSP characteristics were found to remain constant throughout the course of infection. An apparent clonal population structure of parasites seemed to cause outbreaks in small isolated villages. The use of standard molecular epidemiologic methods to measure genetic diversity in malaria revealed the occurrence of a genetically monomorphic population of P. falciparum within a human community.

Plasmodium falciparum: effect of chemical structure on efficacy and specificity of antisense oligonucleotides against malaria in vitro.

Antisense oligodeoxynucleotides (AS ODNs) have shown promise both as potential anti-malarial chemotherapeutic agents and as a means for identifying genes critical for parasite survival. Because conventional ODNs containing phosphodiester (PO) groups are subject to rapid nuclease degradation, ODNs with phosphorothioate (PS) groups are commonly used. However, at high concentration, these lose target specificity, and in some animal models, they become toxic. We compared a variety of chemical modifications (PO, PS, PO-PS hybrids, 2'-O-methyl-2'-deoxy chimeras) and structural modifications (sequence alterations favoring self-stabilizing loop formation) for their ability to inhibit Plasmodium falciparum malaria cultured in vitro. All modifications were done using an AS ODN sequence targeted against dihydrofolate reductase thymidylate synthase (DHFR). Inhibition by PO-PS hybrids containing as few as three PS groups at the 3'- and 5'-ends did not differ significantly from that obtained using compounds containing all-PS groups. Similarly, inhibition by PS chimeric compounds containing 2'-O-methyl modifications did not differ significantly from that of conventional PS compounds. In contrast, while inhibition by PO-PS hybrid chimeras did not differ significantly from that of all-PS compounds at low concentrations, at 1 microM they inhibited parasite growth 25% less (P < 0.001) than all-compounds or PS 2'-O-methyl-2'-deoxy chimeras. Extension of the nucleotide sequence to increase stem-loop formation yielded two compounds which inhibited parasite growth about 20% more than unmodified compounds, though this difference was not significant. Furthermore, most of this increase appears to correlate with the greater number of PS groups associated with the increased ODN length. We conclude that limiting the number of PS groups and inclusion of PO 2'-O-methyl groups may yield compounds with high antisense activity but low non-sequence-dependent effects. Such compounds are currently being tested in vivo.

Inhibition of Plasmodium falciparum malaria using antisense oligodeoxynucleotides.

We studied inhibition of growth of the malaria parasite Plasmodium falciparum in in vitro culture using antisense (AS) oligodeoxynucleotides (ODNs) against different target genes. W2 and W2mef strains of drug-resistant parasites were exposed to AS ODNs over 48 hr, and growth was determined by microscopic examination and [3H]hypoxanthine incorporation. At ODN concentrations of 1 microM, phosphorothioate (PS) ODNs inhibited growth in a target-independent manner. However, between 0.5 and 0.005 microM, ODNs against dihydrofolate reductase, dihydropteroate synthetase, ribonucleotide reductase, the schizont multigene family, and erythrocyte binding antigen EBA175 significantly inhibited growth compared with a PS AS ODN against human immunodeficiency virus, two AS ODNs containing eight mismatches, or the sense strand controls (P < 0.0001). The IC50 was approximately 0.05 microM, whereas that for non-sequence-specific controls was 15-fold higher. PS AS ODNs against DNA polymerase alpha showed less activity than that for other targets, whereas a single AS ODN against triose-phosphate isomerase did not differ significantly from controls. We conclude that at concentrations below 0.5 microM, PS AS ODNs targeted against several malarial genes significantly inhibit growth of drug-resistant parasites in a nucleotide sequence-dependent manner. This technology represents an alternative method for identifying malarial genes as potential drug targets.

Comparison of the polymerase chain reaction and serology for the detection of canine visceral leishmaniasis.

The polymerase chain reaction (PCR) and serology was evaluated for the diagnosis of canine visceral leishmaniasis in Bahia, Brazil in a study of 125 dogs. The PCR was 100% sensitive in 25 dogs that had Leishmania demonstrated by either culture or hamster inoculation. It was 100% specific for 35 dogs from the northeastern United States, all were PCR negative. However, 22 of 54 Brazilian dogs that were culture-hamster inoculation-negative were positive by PCR. The nature of the PCR product was identified by hybridization with specific Leishmania probes. Whereas the sensitivity of serology in relationship to infection, as determined by hamster or culture assay was more than 80%, sensitivity of serology was only 63% when compared with PCR. These results raise questions about the use of serology to detect Leishmania infection in dogs, and suggest that the PCR might serve as a better gold standard to define Leishmania infection than culture or hamster inoculation.

Specificity of diagnostic PCR amplification for M. avium using the probe pMAV22.

We characterized the serovar specificity of the probe pMAV22 using ATCC isolates. Primers from this sequence amplified DNA from serovars 1-6, 8-10, and ATCC strain 19075 but did not amplify MAIS serovars 7, 11, 12, 14, 17-20. This confirmed that the M. avium probe pMAV22 is specific for M. avium, not M. intracellulare.

Comparison of genus- and species-specific probes for PCR detection of mycobacterial infections.

We describe experiments comparing use of different DNA probes to detect mycobacteria in clinical specimens after PCR. The objective was to assess correlation between results using Mycobacterium genus-specific, and species-specific M. tuberculosis probes. Given sufficient concordance, sequential use of such probes would provide a useful screening tool. An evaluation of genus-specific probes compared use of repetitive sequences in the clone pMAv17 with the 65-kDa sequences. Sensitivity was 100% for pMAv17, 93% using the 65-kDa sequence; specificity was 70% for both. We then compared M. tuberculosis-specific probes developed by us (Tb400) with IS6110 and mpt40. Sensitivity using Tb400 was 100%; using IS6110 was 97%, and using mpt40 was 50%. Specificity using Tb400 and IS6110 was 68%, and was 70% using mpt40. Fourteen specimens which were PCR-positive and culture-negative, were positive using both genus probes, and the M. tuberculosis-specific probes Tb400, and IS6110. Ten of these were positive using mpt40.

Plasmodium falciparum and P. vivax: factors affecting sensitivity and specificity of PCR-based diagnosis of malaria.

We have previously reported on development of a simple PCR-based method to detect Plasmodium falciparum in which patient blood samples are lysed and then filtered onto paper. The present studies, conducted in Thailand, were designed to identify factors contributing to differences between results from microscopy and PCR. To analyze microscope-positive, PCR-negative results, we demonstrated that using the lysis/filtration sample preparation method, target DNA degradation is not a significant factor. Similarly, we showed that sensitivity of PCR among patient samples did not differ using the lysis centrifugation method or organically extracted DNA. We further demonstrated that 7/13 samples which were negative by PCR for P. falciparum were positive by PCR when P. vivax-specific primers were used. Microscope-negative, P. falciparum PCR-positive samples were analyzed in two ways: the true rate of false-positivity for PCR (2%) was established by analyzing 498 samples from patients living in areas without transmission. We further showed that when microscope-negative, PCR-positive samples were amplified using an independent P. falciparum-specific PCR target sequence, 42/47 were PCR-positive. We conclude that the accuracy and reduced limit of detection of microscopy are major confounders when comparing this method to PCR.

Use of the polymerase chain reaction to directly detect malaria parasites in blood samples from the Venezuelan Amazon.

We have examined the reproducibility, sensitivity, and specificity of detecting Plasmodium falciparum using the polymerase chain reaction (PCR) and the species-specific probe pPF14 under field conditions in the Venezuelan Amazon. Up to eight samples were field collected from each of 48 consenting Amerindians presenting with symptoms of malaria. Sample processing and analysis was performed at the Centro Amazonico para la Investigacion y Control de Enfermedades Tropicales Simon Bolivar. A total of 229 samples from 48 patients were analyzed by PCR methods using four different P. falciparum-specific probes. One P. vivax-specific probe and by conventional microscopy. Samples in which results from PCR and microscopy differed were reanalyzed at a higher sensitivity by microscopy. Results suggest that microscopy-negative, PCR-positive samples are true positives, and that microscopy-positive and PCR-negative samples are true negatives. The sensitivity of the DNA probe/PCR method was 78% and its specificity was 97%. The positive predictive value of the PCR method was 88%, and the negative predictive value was 95%. Through the analysis of multiple blood samples from each individual, the DNA probe/PCR methodology was found to have an inherent reproducibility that was highly statistically significant.

DNA probes as epidemiological tools for surveillance of Plasmodium falciparum malaria in Thailand.

We have previously reported on development of a DNA probe-based method for diagnosing Plasmodium falciparum infection directly from patient blood samples. In the present studies, we sought to examine applicability of the method to large epidemiological surveys, comparing sensitivity, specificity, time required to obtain results, and costs with those of conventional microscopic examination. Results of DNA probe hybridization were also compared between laboratories in the US and Thailand, to assess transferability of the DNA probe technology. Five separate surveys of approximately 5000 villagers each were performed between December 1987 and June 1989 (26,176 samples total). Sensitivity ranged from 61% to 92% for both US and Thai laboratories, while specificity ranged from 98.2% to 99.9%. Agreement between the US and Thai laboratories was good, with kappa coefficients between 0.62 and 0.78 for different surveys. Between 4 and 8 person-days were required to obtain results from each set of 5000 samples by DNA hybridization, whereas microscopic examination required 150 person-days. Approximate costs were US 0.17 per sample for DNA probe analysis, and US$0.36 for microscopic examination. We conclude that the DNA probe method offers significant advantages when large numbers of samples must be surveyed for P. falciparum.

Use of PCR in the field.

Polymerase chain reaction (PCR) represents a powerful new technology with a variety of field applications. While most of these are still experimental, implementation of PCR-based detection of Onchocerca volvulus in black flies, and subspecific differentiation strongly suggest that potential problems can be overcome. Because of high sensitivity and specificity, PCR provides in some cases the only means to address central parasitological questions, and may well become the 'gold standard' by which other diagnostic techniques are measured. In spite of these advantages, routine implementation of PCR,at present,requires transportation of samples to a central facility for processing, and personnel whose technical competence is high. In addition, reagents are expensive. Robert Barker here weighs up these considerations with regard to the potential utility of PCR assays for some applications.

Diagnosis of Mycobacterium avium bacteremia by polymerase chain reaction.

We describe a rapid polymerase chain reaction (PCR)-based test for diagnosing Mycobacterium avium directly from blood specimens. Blood was collected in anticoagulant (EDTA) from patients who also had blood cultures performed by the lysis-centrifugation method. Blood samples were centrifuged on a Ficoll-Hypaque gradient to purify peripheral blood mononuclear cells. The purified cells were washed and incubated in the presence of Chelex-100 (a divalent cation-binding resin), boiled to release mycobacterial DNA, and then amplified with M. avium-specific PCR primers. Amplification was detected by hybridization with radiolabelled probe, and the results were compared with the culture results. The PCR assay gave positive results for 12 of 15 specimens that were taken from patients with positive cultures for M. avium complex (sensitivity, 80%). The three PCR-negative specimens in this group showed evidence of PCR inhibition. The PCR assay gave positive results for 32 of 228 specimens taken from patients with negative cultures (specificity, 86%). Of these 32 PCR-positive culture-negative specimens, 27 were also positive when amplified with primers specific for the genus Mycobacterium, suggesting that PCR may be more sensitive than culture.

A simple method to detect Plasmodium falciparum directly from blood samples using the polymerase chain reaction.

We have developed a simple method for treating blood samples permitting direct detection of Plasmodium falciparum parasites using the P. falciparum-specific DNA probe pPF14 after polymerase chain reaction (PCR) amplification of target DNA sequences, and have compared this method with microscopic examination of thick blood smears. For PCR amplification, blood samples were lysed, then filtered onto filter paper. After drying, a piece of the filter paper was added directly to the PCR mixture for amplification. The presence of PCR products was detected using nonisotopically labeled probe. This method permits detection of less than than 10 parasites in a 20-microliters sample, and minimizes the effects of PCR inhibitors generally found in blood. Samples were collected from patients presenting at malaria clinics in Mae Sod and Mae Ramat, Thailand, and 626 were analyzed both by the PCR method and by conventional microscopy. Of these, 157 were positive both by microscopy and by PCR, while 297 were negative by both methods. PCR detected 131 samples that were negative by microscopy, and failed to detect 41 samples identified as positive by microscopy. All discordant samples were re-analyzed by microscopy and by PCR. Upon re-examination at a higher sensitivity, microscopy identified five additional positive cases, while six previously positive cases were found to be negative. This method of treating blood samples for PCR may also be useful in other diagnostic assays.

A simple method for diagnosing M. tuberculosis infection in clinical samples using PCR.

Species identification of Mycobacterium tuberculosis remains a cumbersome process. We have developed a simple method for treating clinical samples which permits direct polymerase chain reaction (PCR) amplification of mycobacterial target DNA without organic extraction. Samples were boiled for 30 min in TE-Triton, then were subjected to 30 cycles of amplification using primers derived from the repetitive clone pMTb4 developed by Patel and coworkers (1990; Journal of Clinical Microbiology 28, 513). Specificity of amplification was confirmed by hybridization with a specific probe labelled non-isotopically. In a model system consisting of cultured bacilli, this system routinely allows detection of a single organism in a sample. In preliminary studies examining applicability of this method to 96 clinical samples, 74 were positive by both PCR and mycobacterial culture, and eight were negative by both methods. Fourteen samples were negative by culture and positive by PCR, and none were positive by culture and negative by PCR. These results suggest that the PCR method may provide a sensitive alternative to conventional species-specific diagnostic methods, and that non-isotopic labelling can be used to detect hybridization in this assay.

Plasmodium falciparum: DNA probe diagnosis of malaria in Kenya.

We previously reported isolation of DNA probe which specifically recognizes Plasmodium falciparum and developed a simple method for its use. The sensitivity and specificity of this DNA probe method have now been extensively field tested in comparison with those of conventional microscopic examination of blood films in two separate studies in Malindi, Kenya, involving a total of 1179 patients. In the second study, which used improved techniques, sensitivity of the DNA probe was 89% when compared to microscopy. We conclude that the DNA probe method compares favorably with conventional microscopy in detecting parasite densities as low as 25 parasites per microliter of blood. A significant advantage of the DNA probe method is that it utilizes a standardized procedure which can simultaneously and reproducibly analyze a large number of samples without opportunity for significant reader bias.

Detection of Plasmodium falciparum infection in human patients: a comparison of the DNA probe method to microscopic diagnosis.

We have previously reported the isolation and testing of a DNA probe specific for the detection of Plasmodium falciparum. Field studies to compare the sensitivity and specificity of the DNA probe with that of light microscopy have been performed. In 2 studies in Thailand, 1,397 patients were tested. Microscope slides were prepared in a standard fashion and examined by clinical technicians and expert microscopists. The DNA probe method compares favorably in sensitivity with routine microscopy, detecting parasite densities as low as 40 parasites/microliters blood in the first study and, after modifications, 20-25 parasites/microliters blood in the second. Modifications included the elimination of salt from the lysis buffer, increasing the pH of the lysis buffer, and use of nylon based hybridization membranes instead of nitrocellulose. The DNA probe method offers the advantage of a standardized procedure that can be used in a batchwise fashion on a large number of samples.