Importance of Therapeutic Drug Monitoring of Rifampicin.

Therapeutic Drug Monitoring (TDM) is a routinely practised clinical laboratory technique which aids the clinicians with a clear clinical judgement of the drug therapy and optimize the doses if necessary. Rifampicin is the most important and potent component of first line therapy of tuberculosis (TB). Several factors like age, weight, gender, doses and formulations, gastro-intestinal disorders, ethnicity etc alter the absorption and bioavailability of rifampicin thus altering the drug levels. Low plasma levels of rifampicin may play a plausible role in slow response to therapy, treatment failure or relapse or acquired drug resistance. TB Patients with further complicated conditions like diabetes or HIV are at an increased risk for poor drug absorption and drug-drug interactions. A standard treatment regimen may be inadequate for some cases as the clinical status of patients vary from case to case. TDM can be used as a clinical tool for identifying patients at high risk of treatment failure, delayed response, drug-drug interactions and help optimization of therapy. In the past two decades numerous reports of TDM of anti-tuberculosis drugs have been reported wherein low rifampicin levels have been a major concern. Rifampicin exhibit concentration dependent killing of mycobacteria. A 2 hour post-dose sample approximates the peak plasma rifampicin concentration (Cmax) and is recommended for TDM of rifampicin. An additional 6 hour sample may be collected to distinguish between delayed absorption and malabsorption. Combined with clinical and bacteriological data, TDM can help clinicians treat slow response / complicated TB patients.

Plasma Voriconazole Estimation by HPLC.

Voriconazole, an antifungal drug exhibiting wide inter-individual variability, is an ideal candidate for therapeutic drug monitoring (TDM). The aim of the present study was to standardize a simple, sensitive and rapid high performance liquid chromatography (HPLC) method with ultraviolet detection to determine plasma voriconazole concentration. The HPLC method consisted of a combination of acetonitrile and water (7:3) as mobile phase with 1 ml/min flow rate and detection at 255 nm. Plasma protein precipitation was carried out using perchloric acid and the filtered supernatant was passed through C18 column (250 × 4.6 mm, 5 µm) for the separation of voriconazole. The limit of quantification of voriconazole was 0.2 mg/L. The assay was validated with a linearity of 0.2-15 mg/L and used clinically for TDM in patient samples. The inter-assay precision was below 15 % for routine quality control samples. Weight based voriconazole doses were prescribed to 26 patients for empirical treatment of invasive fungal infections. Voriconazole therapy was managed from the baseline drug levels and follow up analysis reflected achievement in clinical efficacy. Routine TDM of voriconazole may reduce adverse events and improve the treatment response in invasive fungal infections.

Correlation of CYP2C19 genotype with plasma voriconazole levels: a preliminary retrospective study in Indians.

BACKGROUND: Voriconazole is an antifungal drug essentially metabolized by cytochrome P450 (CYP2C19) isozyme. Plasma voriconazole levels exhibit wide inter-individual variability due to several factors like age, weight, food or drug interactions or CYP2C19 polymorphisms. OBJECTIVE: In the present study, we assessed the correlation of voriconazole levels with CYP2C19 genotype in patients on voriconazole therapy. SETTING: Biochemistry Department of a 480 inpatient bed tertiary care hospital in India. METHODS: Plasma voriconazole estimation was done in seventy-two patients on standard weight based voriconazole therapy by High Performance Liquid Chromatography (HPLC) while genotype assessment for the CYP2C19*2 and *3 was done by PCR-RFLP and *17 by ARMS-PCR. Statistical analysis and genotype-phenotype correlation was done by comparing the drug levels with the CYP2C19 genotype. MAIN OUTCOME MEASURE: CYP2C19 polymorphisms influence voriconazole metabolism. RESULTS: A wide variability is seen in plasma levels with only 51% attaining therapeutic levels. The allele frequency of *2, *3 and *17 variant were found to be 33.3, 0.7 and 18% respectively. The drug levels in carriers of *2 allele (poor metabolizers) was twofold higher than that in extensive metabolizers. However, the influence of *2 allele was compromised in presence of *17 allele and patients had low voriconazole levels. In addition to the genotype, co-medication and clinical condition remarkably influenced voriconazole concentration. CONCLUSION: Plasma voriconazole levels are influenced by CYP2C19 variants, drug interactions and clinical condition of the patient. Genotype assessment at initiation of therapy followed by drug monitoring would help optimizing therapeutic efficacy and minimizing toxicity.