Pharmacokinetics of Anti-TB Drugs in HIV/TB Co-infected Children in Ghana

The treatment guidelines for childhood TB are largely inferred from studies in adults, as few controlled trials have been done in children to establish optimum regimens. The standard anti-TB treatment consists of a combination of isoniazid, rifampin, pyrazinamide and ethambutol for two months induction phase followed by 2 months of isoniazid and rifampin in the continuation phase. The plasma and tissue concentrations of each of the drugs in the combination regimen needs to be optimized to maximize bacterial killing and reduce the risk of treatment failure with emergence of drug resistance. Several studies have shown that the peak concentrations of these drugs in a large proportion of children are so low that there is a concern for ineffective therapy in some children. This concern led to the recent recommendation by the WHO to increase the dosages of all first-line anti-TB drugs in children. To our knowledge, the safety and pharmacokinetics of the new dosages have not been studied in children in West Africa. In addition, differences in drug absorption, metabolism and excretion may put some children at risk of low drug concentrations when the standard weight-based fixed-dosages are prescribed to all children. Identification of these individuals clinically or through genetic testing may be important for individualized dosing. The factors that may influence drug concentrations and treatment effects include age, HIV coinfection status, nutritional status and genetic polymorphisms of drug metabolizing enzymes and transporters. Younger children appear to eliminate the drugs faster and have lower plasma peak concentrations compared to older children and adults treated with similar mg/kg doses. The association between nutritional status and the plasma pharmacokinetics of the anti-TB drugs is less clear, as some but not all studies report a significant relationship. HIV coinfection is an important factor that has been associated with poor treatment responses as a result of poor drug absorption.

The primary objective of this study is to evaluate the pharmacokinetics and tolerability of the elevated dosages of the first-line anti-TB in children, as well as factors associated with inter-individual variability in anti-TB drugs exposure in Ghanaian children.

A two-arm pharmacokinetic study in children with active TB with or without HIV coinfection will be performed at Komfo Anokye Teaching Hospital, Kumasi, Ghana. Children aged between 3 months and 14 years, for whom informed consent by parent or guardian have been obtained, will be enrolled. A complete medical history, physical examination, and nutritional status assessment will be performed at enrolment at subsequent study visits. Relevant data will be collected using standardized forms. Baseline measurements prior to initiation of anti-TB treatment will include complete blood count (CBC), blood urea nitrogen, creatinine, liver function tests (LFTs), as well as CD4 cell count determination and plasma HIV-1 RNA level (if HIV co-infected). Measurements LFTs will be repeated at week 2 of therapy or when clinically indicated to evaluate for drug toxicity. All study participants will follow-up at 2 and 4 weeks and then monthly to assess adverse events and clinical response to therapy.

Anti-TB treatment will be initiated immediately upon TB diagnosis and concurrent initiation ART will be allowed in co-infected children as per national and WHO guidelines. The standard anti-TB regimen consists of daily ingestion of isoniazid, rifampin, pyrazinamide and ethambutol for 2 months, then rifampin and isoniazid daily for 4 months. The new WHO recommended doses for children will be prescribed. Weight-based fixed-dose combination tablets are used. Children will receive directly observed therapy for the anti-TB treatment by healthcare worker at the hospital or family member at home. Treatment outcome (completed/cured, died, defaulted, discontinued, transferred out) is defined according to WHO criteria.

Pharmacokinetic sampling will be performed at 4 weeks of anti-TB therapy. Study drugs will be administered after at least a 2-hour fast in non-breastfed children and blood samples obtained through intravascular catheters at times 0, 2, 4, 6, 8-hours post dosing. Actual times of dosing and sampling will be accurately recorded. Doses vomited < 30 minutes after dosing will require cancellation of PK testing and rescheduling. The samples will be placed immediately on ice and centrifuged within 30 minutes at 3000g for 10 minutes at 4oC. Plasma will be stored at - 70oC until measurement of drug concentrations. Drug concentrations will be determined using validated gas chromatography with mass spectrometry.