Therapeutic efficacy and macrofilaricidal activity of doxycycline for the treatment of river blindness

Abstract

Background: Onchocerca volvulus and lymphatic filariae, causing river blindness and elephantiasis, depend on endosymbiotic Wolbachia bacteria for growth, development, fertility, and survival. Clinical trials have shown that doxycycline treatment eliminates Wolbachia, causing long-term sterilization of adult female filariae and effecting potent macrofilaricidal activity. The continual reinfection by drug-naive worms that occurs in these trial settings dilutes observable anti-Wolbachia and antifilarial effects, making it difficult to estimate therapeutic efficacy and compare different doxycycline regimens, evaluated at different times after treatment.

Methods: A meta-analytical modeling framework is developed to link all usable data collected from clinical trials measuring the Wolbachia status and viability of individual female adult worms collected at various times after treatment with 4, 5, or 6 weeks of daily 100 or 200 mg oral doxycycline. The framework is used to estimate efficacy parameters that are not directly measurable as trial outcomes.

Results: The estimated efficacy of doxycycline (the maximum proportional reduction in the percentage of adult female O. volvulus positive for Wolbachia) is 91%-94% on average, irrespective of the treatment regimen. Efficacy is >95% in the majority of trial participants. The life span of Wolbachia-depleted worms is reduced by 70%-80%, from approximately 10 years to 2-3 years.

Conclusions: The efficacy parameters are pertinent to the prospects of using doxycycline on a "test and treat" basis for onchocerciasis control and confirm doxycycline as a potent macrofilaricidal therapy. The modeling approach is more generally relevant to the design and evaluation of clinical trials for antifilarial drugs conducted in endemic settings.

Keywords: clinical trials; doxycycline; efficacy; macrofilaricide; onchocerciasis.

Figure 1.

Schematic representation of the model describing the dynamics of Wolbachia depletion from adult filariae coupled to the ensuing antifilarial effects. The variables L_i(t), W_i⁺(t), W_i^± (t), W_i^–(t), and D_i(t) denote, respectively, the mean number of infective (L3) larvae, and Wolbachia-positive, Wolbachia-depleted, Wolbachia-negative, and adult dead worms in trial participants given drug regimen i. The smaller gray squares in (A) indicate the positions of latent compartments used to assess model structural uncertainty on parameter estimates. These latent compartments are omitted from (B) and (C) for clarity. Temporal dependence in certain per worm rate parameters [eg, δ_i(t)] is governed by whether the average blood plasma concentration of doxycycline in participants given drug regimen i, C_i(t), is greater than the minimum inhibitory concentration (MIC) against Wolbachia (Supplementary Methods, Pharmacokinetic Model). Infective, L3 larvae are acquired at rate Λ, die at (per capita) rate σ_i(t), and progress toward Wolbachia-positive adult worms at rate γ_i(t). When the concentration of doxycycline is above the MIC, σ_i(t) = σ₀ + σ₁ and γ_i(t) = 0 (B). Below the MIC, σ_i(t) = σ₀ and γ_i(t) = γ₀. Wolbachia-positive worms die at rate µ₀, becoming immunohistologically or morphologically identifiable as dead, before being resorbed at rate η. Depletion of Wolbachia occurs at rate δ_i(t) = δ₁ while the concentration of doxycycline is greater than the MIC; no depletion occurs otherwise [δ_i(t) = 0]. Wolbachia-depleted worms become immunohistologically observable as Wolbachia negative at rate ζ. Both Wolbachia-depleted and Wolbachia-negative worms incur a higher mortality rate than their Wolbachia-positive counterparts, µ₀ + µ₁ (the combination of background mortality and doxycycline-induced, excess mortality).

Figure 2.

Fitted and observed proportions of female Onchocerca volvulus by Wolbachia status and viability, against time since the start of doxycycline treatment. The proportion of (live) Wolbachia-positive female worms is depicted in (A) and the proportion of (total) live female worms is depicted in (B). In each panel, the thick blue, green, and red lines denote, respectively, the marginal percentage of worms (ie, averaged over trial participants) receiving doxycycline for 4, 5, or 6 weeks respectively. Solid and broken lines indicate doses of 100 mg and 200 mg, respectively. The thick solid gray line denotes the marginal percentage of worms in placebo-treated and untreated control patients (Table 1). Thin lines, applying the same color scheme to indicate different treatment regimens, represent (posterior mean) individual patient trajectories. Variation among individual trajectories in the percentage of (total) live worms and the percentage of (live) Wolbachia-positive worms is governed by the precision parameters υ₁ and υ₂ (Table 2), respectively. Data points represent the observed data grouped (for presentation purposes only; the model is fitted to the individual patient data) by follow-up times (follow-up times disaggregated by month of follow-up are given in Supplementary Table 1) and plotted at the median follow-up time per group. The color of the data points corresponds to the duration of treatment in the same manner as the model-predicted proportions. A treatment dose of 100 mg or 200 mg per day is indicated by a circle or a triangle, respectively; data from untreated or placebo treated patients are indicated by (gray) squares. Vertical bars represent exact (likelihood profile) 95% confidence intervals for the observed data. Note that data from different regimens collected at proximate times are very similar, verifying that different treatment regimens are of approximately equivalent efficacies (Figure 3). The progressive increase in the proportion of live, Wolbachia-positive worms after 9.5 months is not due to recrudescence of the bacteria, but to both acquisition of new, drug-naive worms with a full complement of bacteria, and increased mortality of treated worms.

Figure 3.

The efficacy of doxycycline in depleting Wolbachia from adult female Onchocerca volvulus estimated for different drug regimens. Efficacy is defined as the maximum proportional reduction in the percentage of adult female O. volvulus positive for Wolbachia. The circles represent posterior medians of the estimates for each of the 114 treated trial participants. These are overlaid with a box-and-whisker plot depicting the interquartile range (IQR) of individual estimates (box) and extending to 1.5 × IQR (whiskers). The solid horizontal bar within the box depicts the median of the individual efficacy estimates. The solid horizontal bars beside the individual estimates represent the marginal efficacy averaged over all patients receiving a particular treatment regimen. The accompanying error bars represent 95% Bayesian credible intervals (BCIs). The values and BCIs of the marginal efficacies are as follows: 100 mg daily for 5 weeks, 93% (89%–96%); 100 mg daily for 6 weeks, 93% (91%–95%); 200 mg daily for 4 weeks, 93% (88%–96%); 200 mg daily for 6 weeks, 91% (86%–94%).

Figure 4.

The distribution of efficacies estimated for individual patients aggregated by treatment regimen. Efficacy is defined as the maximum proportional reduction in the percentage of adult female Onchocerca volvulus positive for Wolbachia. Efficacy estimates represent posterior medians for each of the 114 treated patients. The vertical dashed line is the median of the individual estimates indicating that the efficacy of doxycycline is > 95% in the majority of patients.