Pharmacogenetic testing for NAT2 genotypes in a Tanzanian population across the lifespan to guide future personalized isoniazid dosing

Abstract

Despite updated recommendations for weight-based isoniazid dosing in children with drug-susceptible tuberculosis (TB) and higher dose isoniazid in regimens for adults with drug-resistant TB, individual pharmacokinetic variability can lead to sub-target isoniazid exposure. Host pharmacogenetics and isoniazid exposure remain understudied, especially in the East African population. We therefore employed a real-time polymerase chain reaction (qPCR) assay system to test genomic DNA extracted from saliva samples targeting the NAT2 gene responsible for isoniazid metabolism to describe the frequency of human single nucleotide polymorphisms in NAT2 within populations of children and adults in Tanzania, ascribe those polymorphisms to acetylator phenotype, and correlate to serum isoniazid exposures. In adults treated with higher dose isoniazid, genotypes with a predicted allelic phenotype of slow or intermediate acetylation were able to achieve a 0.41 μg/mL higher C_max (p = 0.018) and a 2.9h*μg/mL higher AUC_0-12 (p = 0.003) per mg/kg increase in isoniazid dosage versus adults with rapid acetylation phenotype. A similar relationship was not found in the younger age population as predicted by timing of NAT2 maturation. This saliva based qPCR assay was fieldable to guide personalized isoniazid dosing in adults but not young children that may not have full NAT2 maturation and activity.

Keywords: Acetylation; Isoniazid; N-acetyltransferase; Pharmacogenetics; Pharmacokinetics.

Fig. 1.. INH exposure analyzed by genotypes at three NAT2 loci.

Available average C_max (μg/mL) and average AUC_0–12 (h*μg/mL) values were analyzed by genotype at each of three NAT2 SNP loci. Each marker represents an individual participant. Those having a non-mature NAT2 phenotype as measured by age less than 5.3 years of age (“Age Category 1”) represented by red markers and blue markers representing individuals 5.3 years of age or older (“Age Category 2”). A) C_max; B) AUC_0–12. The red, dashed line in each subplot represents the target expected exposure level for adult patients.

Fig. 2.. INH exposure analyzed by predicted NAT2 acetylator phenotype.

Genotype information at c.341 and c.590 were converted to NAT2 haplotypes/alleles and predicted NAT2 acetylator phenotypes and then used to analyze available average C_max (μg/mL) and average AUC_0–12 (h*μg/mL) values. Data was further stratified according to the same age categories used in Fig. 1. Each marker represents an individual participant. A) C_max; B) AUC_0–12. The red, dashed line in each subplot represents the target expected exposure level for adult patients.

Fig. 3.. INH exposure analyzed by mg/kg dosage and predicted NAT2 acetylator phenotype.

Average C_max (μg/mL) and average AUC_0–12 (h*μg/mL) values were first stratified by predicted NAT2 acetylator phenotype and then plotted against the averaged mg/kg INH dosage given to each patient. Each marker represents an individual participant with individuals with a non-mature NAT2 phenotype as measured by age less than 5.3 years of age (“Age Category 1”) represented by red markers and blue markers representing individuals 5.3 years of age or older (“Age Category 2”). A) C_max; B) AUC_0–12. The red, dashed line in each subplot represents the target expected exposure level for adult patients.