Towards optimal design of anti-malarial pharmacokinetic studies

Antimalarial treatment in infants

INTRODUCTION

Malaria in infants is common in high-transmission settings, especially in infants >6 months. Infants undergo physiological changes impacting pharmacokinetics and pharmacodynamics of anti-malarial drugs and, consequently, the safety and efficacy of malaria treatment. Yet, treatment guidelines and evidence on pharmacological interventions for malaria often fail to address this vulnerable age-group. This review aims to summarise the available data on anti-malarial treatment in infants.

AREAS COVERED

The standard recommended treatments for severe and uncomplicated malaria are generally safe and effective in infants. However, infants have an increased risk of drug-related vomiting and have distinct pharmacokinetic parameters of antimalarials compared with older patients. These include larger volumes of distribution, higher clearance rates and immature enzyme systems. Consequently, infants with malaria may be at increased risk of treatment failure and drug toxicity.

EXPERT OPINION

Knowledge expansion to optimize treatment can be achieved by including more infants in antimalarial drug trials and by reporting separately on treatment outcomes in infants. Additional evidence on the efficacy, safety, tolerability, acceptability and effectiveness of ACTs in infants is needed, as well as population pharmacokinetics studies on antimalarials in the infant population.

Twice neglected? Neglected diseases in neglected populations

It is unfortunately true that clinicians lack the necessary evidence to know how to use medications properly in large sections of the population and do not have optimal treatments to use for many neglected tropical diseases (NTDs). NTDs often disproportionately affect neglected populations that are left out of research efforts, such as children and pregnant women. As reliable access to safe, effective preventives and treatments can break the cycle of poverty, illness, and ensuing debility that further perpetuates poverty, it is of paramount importance to investigate and develop new medicines for neglected populations suffering from NTDs. Furthermore, there is not only a need to develop and evaluate novel therapies, but also to ensure that these are affordable, available, and adapted to the communities who need them. The NIH has proposed a "4 C's" framework which is relevant for neglected diseases and populations and should be leveraged for the study of the Twice Neglected: Consider inclusion; Collect data from neglected populations with neglected conditions; Characterize differences through meaningful analysis; Communicate findings pertaining to neglected diseases and populations. With this editorial, the British Journal of Clinical Pharmacology hereby launches a call for high-quality articles focusing on NTDs in special populations, to facilitate and encourage the reversal of this dual neglect.

Pharmacokinetic profile of amodiaquine and its active metabolite desethylamodiaquine in Ghanaian patients with uncomplicated falciparum malaria

BACKGROUND

Accurate measurement of anti-malarial drug concentrations in therapeutic efficacy studies is essential to distinguish between inadequate drug exposure and anti-malarial drug resistance, and to inform optimal anti-malarial dosing in key target population groups.

METHODS

A sensitive and selective LC-MS/MS method was developed and validated for the simultaneous determination of amodiaquine and its active metabolite, desethylamodiaquine, and used to describe their pharmacokinetic parameters in Ghanaian patients with uncomplicated falciparum malaria treated with the fixed-dose combination, artesunate-amodiaquine.

RESULTS

The day-28 genotype-adjusted adequate clinical and parasitological response rate in 308 patients studied was > 97% by both intention-to-treat and per-protocol analysis. After excluding 64 patients with quantifiable amodiaquine concentrations pre-treatment and 17 with too few quantifiable concentrations, the pharmacokinetic analysis included 227 patients (9 infants, 127 aged 1-4 years, 91 aged ≥ 5 years). Increased median day-3 amodiaquine concentrations were associated with a lower risk of treatment failure [HR 0.87 (95% CI 0.78-0.98), p = 0.021]. Amodiaquine exposure (median AUC0-∞) was significantly higher in infants (4201 ng h/mL) and children aged 1-5 years (1994 ng h/mL) compared to older children and adults (875 ng h/mL, p = 0.001), even though infants received a lower mg/kg amodiaquine dose (median 25.3 versus 33.8 mg/kg in older patients). Desethylamodiaquine AUC0-∞ was not significantly associated with age. No significant safety concerns were identified.

CONCLUSIONS

Efficacy of artesunate-amodiaquine at currently recommended dosage regimens was high across all age groups. Reassuringly, amodiaquine and desethylamodiaquine exposure was not reduced in underweight-for-age young children or those with high parasitaemia, two of the most vulnerable target populations. A larger pharmacokinetic study with close monitoring of safety, including full blood counts and liver function tests, is needed to confirm the higher amodiaquine exposure in infants, understand any safety implications and assess whether dose optimization in this vulnerable, understudied population is needed.

Absence of gender influence on the pharmacokinetics of chloroquine combined with primaquine in malaria vivax patients

Chloroquine is the first-line therapy against the asexual stages of Plasmodium vivax . There is a high variation of chloroquine plasma levels after therapeutic doses, which can lead to inadequate exposure to the drug. The gender influence was low regarding the disposition of the drug, which is relevant as there are significant physiological variations between male and female patients. The objective of the study was to investigate whether gender modifies the pharmacokinetics parameters of chloroquine in patients with malaria vivax. A prospective study was performed in male and female adult patients using chloroquine (total dose of 25 mg/kg for three days) combined with primaquine. Serial blood samples were collected at admission and up to 672 h post-administration of the drugs. Chloroquine was measured in plasma samples by high-performance liquid chromatography with fluorescence detection. A non-compartmental analysis was used for modeling the data. A total of 26 male and 25 female patients were enrolled in the study. The pharmacokinetic parameters of chloroquine were similar between male and female patients: a half-life of 9.5 days and 10.2 days, maximum concentration (Cmax) of 1295 ng/ml and 1220 ng/ml, area-under-the-curve (AUC 0–28) of 241 µg/mL h and 237 µg/mL h, observed clearance (CL/f) of 5.8 and 5.5 L/h and the volume of distribution (V/f) of 1869 L and 1936 L. The study results suggest that a similar dose regimen of chloroquine combined with primaquine provides a comparable pattern of exposure in male and female patients.

Population pharmacokinetics of levofloxacin in Korean patients

Levofloxacin (LVFX) has different effects depending on the area under the concentration-time curve (AUC)/minimum inhibitory concentration (MIC) ratio. While AUC can be expressed as dose/clearance (CL), we measured serial concentrations of LVFX in Koreans and tried to set a Korean-specific equation, estimating the CL of the antibiotic. In total, 38 patients, aged 18-87 years, received once daily intravenous LVFX doses of 500 mg or 250 mg, depending on their renal function. Four plasma samples were obtained according to a D optimal sampling design. The population pharmacokinetic (PK) parameters of LVFX were estimated using non-linear mixed-effect modeling (NONMEM, ver. 7.2). The CL of LVFX was dependent on creatinine clearance (CLCR) as a covariate. The mean population PK parameters of LVFX in Koreans were as follows: CL (l/hour) = 6.19 ×  (CLCR/75)(1.32). The CL of LVFX in Koreans is expected to be lower than that in Western people.

Designing a Pediatric Study for an Antimalarial Drug by Using Information from Adults

The objectives of this study were to design a pharmacokinetic (PK) study by using information about adults and evaluate the robustness of the recommended design through a case study of mefloquine. PK data about adults and children were available from two different randomized studies of the treatment of malaria with the same artesunate-mefloquine combination regimen. A recommended design for pediatric studies of mefloquine was optimized on the basis of an extrapolated model built from adult data through the following approach. (i) An adult PK model was built, and parameters were estimated by using the stochastic approximation expectation-maximization algorithm. (ii) Pediatric PK parameters were then obtained by adding allometry and maturation to the adult model. (iii) A D-optimal design for children was obtained with PFIM by assuming the extrapolated design. Finally, the robustness of the recommended design was evaluated in terms of the relative bias and relative standard errors (RSE) of the parameters in a simulation study with four different models and was compared to the empirical design used for the pediatric study. Combining PK modeling, extrapolation, and design optimization led to a design for children with five sampling times. PK parameters were well estimated by this design with few RSE. Although the extrapolated model did not predict the observed mefloquine concentrations in children very accurately, it allowed precise and unbiased estimates across various model assumptions, contrary to the empirical design. Using information from adult studies combined with allometry and maturation can help provide robust designs for pediatric studies.

Modelling the time course of antimalarial parasite killing: a tour of animal and human models, translation and challenges

Malaria remains a global public health concern and current treatment options are suboptimal in some clinical settings. For effective chemotherapy, antimalarial drug concentrations must be sufficient to remove completely all of the parasites in the infected host. Optimized dosing therefore requires a detailed understanding of the time course of antimalarial response, whilst simultaneously considering the parasite life cycle and host immune elimination. Recently, the World Health Organization (WHO) has recommended the development of mathematical models for understanding better antimalarial drug resistance and management. Other international groups have also suggested that mechanistic pharmacokinetic (PK) and pharmacodynamic (PD) models can support the rationalization of antimalarial dosing strategies. At present, artemisinin-based combination therapy (ACT) is recommended as first line treatment of falciparum malaria for all patient groups. This review summarizes the PK-PD characterization of artemisinin derivatives and other partner drugs from both preclinical studies and human clinical trials. We outline the continuous and discrete time models that have been proposed to describe antimalarial activity on specific stages of the parasite life cycle. The translation of PK-PD predictions from animals to humans is considered, because preclinical studies can provide rich data for detailed mechanism-based modelling. While similar sampling techniques are limited in clinical studies, PK-PD models can be used to optimize the design of experiments to improve estimation of the parameters of interest. Ultimately, we propose that fully developed mechanistic models can simulate and rationalize ACT or other treatment strategies in antimalarial chemotherapy.

Pharmacokinetic and pharmacodynamic considerations in antimalarial dose optimization

Antimalarial drugs have usually been first deployed in areas of malaria endemicity at doses which were too low, particularly for high-risk groups such as young children and pregnant women. This may accelerate the emergence and spread of resistance, thereby shortening the useful life of the drug, but it is an inevitable consequence of the current imprecise method of dose finding. An alternative approach to dose finding is suggested in which phase 2 studies concentrate initially on pharmacokinetic-pharmacodynamic (PK-PD) characterization and in vivo calibration of in vitro susceptibility information. PD assessment is facilitated in malaria because serial parasite densities are readily assessed by microscopy, and at low densities by quantitative PCR, so that initial therapeutic responses can be quantitated accurately. If the in vivo MIC could be characterized early in phase 2 studies, it would provide a sound basis for the choice of dose in all target populations in subsequent combination treatments. Population PK assessments in phase 2b and phase 3 studies which characterize PK differences between different age groups, clinical disease states, and human populations can then be combined with the PK-PD observations to provide a sound evidence base for dose recommendations in different target groups.

Dried blood spots in bioanalysis of antimalarials: relevance and challenges in quantitative assessment of antimalarial drugs

Malaria is the leading parasitic disease in emerging countries. Therapeutic drug monitoring of antimalarial drugs is becoming increasingly important due to their spreading resistance. Measuring systemic antimalarial drug concentrations is also vital for safety and PK evaluations during clinical development. The dried blood spot (DBS) technique is a convenient alternative sample-collection method to venipuncture, especially in resource -limited areas where the clinical studies of antimalarials are usually carried out. Various bioanalytical methods for antimalarial drug estimation utilizing DBS sampling have been reported. This review discusses the applicability and relevance of DBS in quantitative assessment of antimalarial drugs, the advantages and drawbacks of DBS, and the difficulties encountered during its implementation.

Improving pharmacokinetic-pharmacodynamic modeling to investigate anti-infective chemotherapy with application to the current generation of antimalarial drugs

Mechanism-based pharmacokinetic-pharmacodynamic (PK/PD) modelling is the standard computational technique for simulating drug treatment of infectious diseases with the potential to enhance our understanding of drug treatment outcomes, drug deployment strategies, and dosing regimens. Standard methodologies assume only a single drug is used, it acts only in its unconverted form, and that oral drugs are instantaneously absorbed across the gut wall to their site of action. For drugs with short half-lives, this absorption period accounts for a significant period of their time in the body. Treatment of infectious diseases often uses combination therapies, so we refined and substantially extended the PK/PD methodologies to incorporate (i) time lags and drug concentration profiles resulting from absorption across the gut wall and, if required, conversion to another active form; (ii) multiple drugs within a treatment combination; (iii) differing modes of action of drugs in the combination: additive, synergistic, antagonistic; (iv) drugs converted to an active metabolite with a similar mode of action. This methodology was applied to a case study of two first-line malaria treatments based on artemisinin combination therapies (ACTs, artemether-lumefantrine and artesunate-mefloquine) where the likelihood of increased artemisinin tolerance/resistance has led to speculation on their continued long-term effectiveness. We note previous estimates of artemisinin kill rate were underestimated by a factor of seven, both the unconverted and converted form of the artemisinins kill parasites and the extended PK/PD methodology produced results consistent with field observations. The simulations predict that a potentially rapid decline in ACT effectiveness is likely to occur as artemisinin resistance spreads, emphasising the importance of containing the spread of artemisinin resistance before it results in widespread drug failure. We found that PK/PD data is generally very poorly reported in the malaria literature, severely reducing its value for subsequent re-application, and we make specific recommendations to improve this situation.

Optimal designs for population pharmacokinetic studies of oral artesunate in patients with uncomplicated falciparum malaria

Background

Currently, population pharmacokinetic (PK) studies of anti-malarial drugs are designed primarily by the logistical and ethical constraints of taking blood samples from patients, and the statistical models that are fitted to the data are not formally considered. This could lead to imprecise estimates of the target PK parameters, and/or designs insufficient to estimate all of the parameters. Optimal design methodology has been developed to determine blood sampling schedules that will yield precise parameter estimates within the practical constraints of sampling the study populations. In this work optimal design methods were used to determine sampling designs for typical future population PK studies of dihydroartemisinin, the principal biologically active metabolite of oral artesunate.

Methods

Optimal designs were derived using freely available software and were based on appropriate structural PK models from an analysis of data or the literature and key sampling constraints identified in a questionnaire sent to active malaria researchers (3-4 samples per patient, at least 15 minutes between samples). The derived optimal designs were then evaluated via simulation-estimation.

Results

The derived optimal sampling windows were 17 to 29 minutes, 30 to 57 minutes, 2.5 to 3.7 hours and 5.8 to 6.6 hours for non-pregnant adults; 16 to 29 minutes, 31 minutes to 1 hour, 2.0 to 3.4 hours and 5.5 to 6.6 hours for designs with non-pregnant adults and children and 35 to 59 minutes, 1.2 to 3.4 hours, 3.4 to 4.9 hours and 6.0 to 8.0 hours for pregnant women. The optimal designs resulted in acceptable precision of the PK parameters.

Conclusions

The proposed sampling designs in this paper are robust and efficient and should be considered in future PK studies of oral artesunate where only three or four blood samples can be collected.