Introduction

This chapter profiles baseline characteristics of the study cohort and events during TB treatment in order to identify clinical factors which independently predict treatment response or are important covariates for pharmacodynamic modelling. Final outcomes for study patients are reviewed and range of factors which may influence the effectiveness of TB therapy are assessed.

Firstly, socio-economic status is discussed. Globally, there is a well established link between TB prevalence and urban poverty473-475 but it is plausible that socio-economic factors are also implicated in treatment outcomes. This is explored using information collected from patients at baseline interview.

Secondly, HIV parameters are outlined to provide information on patients’ HIV status and describe access to ART during the study. Although HIV does not influence cure from PTB476,477 more relapses are reported in HIV-infected individuals478,479. The specific effect of ART on TB outcome is unknown but emerging evidence suggests that it reduces total mortality122,123 and Malawian national policy has recently changed from stating that naive patients should initiate ART during the continuation phase of TB therapy464, to

recommending initiation “within 14 days of TB diagnosis”465. As noted in Section 2.2.2, in September 2011 standard first-line ART regimen for TB patients was changed from

stavudine, lamivudine and nevirapine to tenofovir, lamivudine and efavirenz. Assessing the effect of these changes when modelling bacillary persistence and elimination is not

straightforward but a detailed description of HIV treatment amongst TB patients is provided to contextualise the analysis.

Thirdly, clinical factors are reviewed, several of which have previously been associated with treatment response including low baseline BMI, poor weight gain during treatment363, ever smoking cigarettes291 and BCG vaccination480. The strength of the evidence for these associations is mixed and prior studies are difficult to compare because they are of differing size, and use varying end-points of two month smear or culture conversion rather than final clinical cure or failure/relapse. Analysis of the current dataset is done to clarify the relative importance of these factors as covariates for the study cohort.

66 Fourthly, serum Vitamin D concentrations of study patients are assessed. Previous work has shown variable baseline Vitamin D deficiency in Malawian adult TB patients481. This may be relevant for two reasons. 1,25 (OH) D, the active metabolite, is an immunologically active hormone which induces anti-mycobacterial activity in vitro482 and modulates the host response to mycobacterial infection by induction of reactive nitrogen and oxygen

intermediates483,484 and the antimicrobial peptide cathelicidin485-487. It has been proposed that these effects of Vitamin D may usefully augment TB treatment and whilst a recent clinical trial of did not show improved outcomes with Vitamin D supplementation, it demonstrated a trend towards faster bacillary clearance from sputum particularly in patients with a tt Vitamin D receptor genotype488,489. From a related perspective, it is necessary to evaluate whether varying Vitamin D levels in a generally deficient population have consequences for treatment response.

Additionally, some components of TB therapy and ART can pharmacologically lower serum concentrations of useful Vitamin D metabolites (Figure 3.1). Vitamin D is normally absorbed from the skin and diet and converted by sequential hydroxylation into 25 (OH) D (the routinely measured compound in serum) then 1,25 (OH) D (the active compound). Isoniazid inhibits both hydroxylation steps490 and rifampicin induces activity of enzymes which degrade 25 (OH) D into an inactive waste product491. Combined rifampicin and isoniazid therapy may reduce serum levels of useful Vitamin D metabolites by 23-34%491. The anti- retroviral drug, efavirenz has also been associated with Vitamin D deficiency492. Although only recently introduced to first-line ART in Malawi, this drug is in the same class as nevirapine which is ubiquitously used, and may have similar metabolic effects. It is important to know whether Vitamin D levels are progressively compromised by drug therapy in a population at high risk of baseline deficiency.

Finally, radiological extent of TB disease is described. CXR abnormalities (especially cavitation) are associated with delayed therapeutic response354,356,358 and clinical trials often use CXR appearance as a covariate in end-point analysis225,228,232. However, many CXRs assessment schemes493,494 are complex, vulnerable to inter-reader variability495 and poorly validated to predict outcome. In 2010, investigators in Papua, Indonesia reported that a simple scoring system based on percentage of lung affected and presence of cavitation predicted 2 month sputum smear conversion in a predominantly HIV-negative population355. This approach is used here to evaluate the significance of CXR abnormalities in a setting of higher HIV endemicity.

67 *25 (OH) Vitamin D is the routinely measured metabolite, commonly used as a surrogate measure of the active compound 1,25 (OH) Vitamin D.

After profiling each aspect of the study cohort, the relevance of each clinical and

radiological covariate to treatment outcome is analysed in relation to binary measures of response. Two month sputum culture status is used as an early marker and final clinical outcome is used as a definitive end-point. The covariates described here will be re-visited in Chapter 6 and 7 to assess their relationship with variability in the parameters of pharmacodynamic and pharmacokinetic models.

Figure 3.1 Vitamin D metabolism and the effect of TB and HIV drugs

RIFAMPICIN and EFAVIRENZ induced hepatic enzymes (CYP24 and CYP3A4) have 24,25 hydroxylase activity 1 α-hydroxylase in the KIDNEY 25-hydroxylase in the LIVER 24,25- hydroxylase In the LIVER 7-dehydrocholesterol in SKIN Ultraviolent sunlight Vitamin D2 from diet 25-OH Vitamin D* 1,25-OH Vitamin D (Active metabolite)

Binds Vitamin D receptor in target tissues

Regulates calcium and bone biochemistry

Exerts immunological effects including anti-mycobacterial activity

24,25-OH Vitamin D (inactive metabolite) ISONIAZID and EFAVIRENZ inhibit cytochrome P-450 system reducing 25-hydroxylase activity

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