In vivo studies are used to assess the clinical efficacy of antimalarial drugs. Patients with malaria illness are enrolled in a study and followed at regular intervals. The World Health Organization (WHO) updates the standard protocol as consensus evolves about the study methodology and end points. According to the current protocol, enrollment occurs on day 0 and follow-up visits take place on days 1, 2, 3, 7, 14, 21, and 28 and any time the patient is ill [1
]. Some authorities suggest that longer follow-up is more sensitive in detecting low levels of resistance by capturing episodes of recurrent parasitemia that occur more than 4 weeks after the treatment episode [2
A successful treatment outcome in a drug efficacy study is adequate clinical and parasitologic response. Failure to have a significant decrease in parasite density or persistent fever in the presence of parasites during the first 3 days of treatment is classified as early treatment failure. Recurrent detectable parasitemia from day 4 until the study’s end is classified as late treatment failure. Treatment failure that occurs with symptomatic infection, usually fever, is often referred to as clinical failure to distinguish it from parasitologic failure that does not take into account the patient’s symptoms.
Recent revisions to the efficacy study procedures have focused on distinguishing recrudescent from new infections in episodes of recurrent parasitemia that occur during the follow-up period. After treatment, the parasites in the blood can fall below the level of detection by microscopy but a small number of resistant parasites may continue to replicate and grow to a detectable level. This is considered a recrudescent infection. In contrast, a malaria infection may be due to a new infectious bite that occurred during the follow-up period. These are differentiated by genotyping of highly polymorphic genes encoding merozoite surface protein (msp
) 1 and 2 and the glutamine-rich protein (glurp
). Recrudescent infections are considered “true” treatment failures because the parasites survived antimalarial therapy, whereas individuals who experience new infections are treated for their infection and censored from follow-up. The WHO has recently determined that polymerase chain reaction (PCR)—correcting drug efficacy outcomes (using genotyping to distinguish recrudescent from new infections) should be included in reported results [3
Conducting a drug efficacy study is a demanding undertaking. It requires devoted personnel to interact with patients, accurately quantify parasitemia density, and follow up patients who fail to return to the clinic for their scheduled visits. Complete results also require PCR analysis. Health infrastructure, budget, and human resources in malaria-endemic countries cannot support frequent assessments or trials in multiple locales. This is problematic because efficacy changes (usually decreases) over short time periods and may vary at different locations in a single country.
Even with a well-performed clinical study, treatment failures may be due to mechanisms other than intrinsic parasite resistance. In addition to the drug’s effect on the parasites, immunologic and pharmacokinetic factors contribute to the observed outcome. The host immune response plays an important role in removing malaria parasites from the blood. Individuals with pre-existing immunity to malaria may successfully clear parasites that are resistant to the administered drug. Because immunity is acquired with increasing exposure to blood-stage parasites, in malaria-endemic areas immunity is directly related to age. As a result, the age of the study participants is critical for interpreting the efficacy of the antimalarial drug under investigation. Older children and adults who have experienced malaria infections throughout their lives demonstrate better responses in drug efficacy studies than younger children because their immune response is able to clear even resistant parasites [4
]. Efforts to prevent malaria transmission (eg, using bednets and indoor residual spraying) may mean less exposure to infection and may slow the acquisition of immunity. Less immunity within a population can lead to an observed decrease in drug efficacy at the same level of intrinsic resistance [7
]. Other host factors can influence drug efficacy. For example, the efficacy of the antifolate combination sulfadoxine-pyrimethamine varies with blood folate levels [8
The interaction between HIV and malaria has been difficult to discern and is beyond the scope of this review [8
]. However, efficacy assessments conducted in populations with high prevalence of HIV infection can yield misleading results. Drug efficacy studies are conducted in patients who have symptomatic malaria infection. In areas of moderate to high malaria transmission, individuals with HIV infection may have asymptomatic parasitemia while experiencing symptoms of an opportunistic infection that are incorrectly attributed to malaria [10
]. Patients may meet the criteria for clinical treatment failure because the underlying cause of the symptoms, such as bacteremia, pneumonia, or viral infection, has not been addressed.
Variability of drug levels may also influence efficacy study outcomes. Inconsistencies in bioavailability and metabolism due to poor-quality drug, interpatient variability in metabolism, and incorrect dosing can lead to differences in the drug level achieved in patients and thus the amount of drug to which the parasites are exposed. Lumefantrine, the non-artemisinin partner drug to artemether in a commonly administered ACT, has best bioavailability when administered with food containing fat and adequate absorption is a key factor in determining the drug efficacy. Lumefantrine level on day 7 is an independent predictor of treatment outcome [11••
]. Some researchers have proposed including a measurement of the drug level on day 7 as a standard part of the drug efficacy study [13
Despite the above-mentioned caveats, drug efficacy studies yielded relatively straightforward results for monotherapy for treating malaria. However, in the era of ACTs, the interpretation of outcomes has become more complex. Treatment failure may reflect poor efficacy of the artemisinins, the partner drug, or both. This has important public health implications because of the limited repertoire of antimalarial medications. Without information about each component, if a combination therapy fails, both drugs may be removed from the national treatment policy, leaving less desirable alternative medications.
Clinical resistance to artemisinins is extremely rare. Although not yet fully characterized, evidence exists of decreased efficacy on the Thai-Cambodian border [14
]. Initially, investigators in the region noted decreased efficacy of the artesunate-mefloquine combination. To investigate further the contribution of artemisinins resistance to the decreased efficacy, studies are being conducted using a 7-day course of artesunate monotherapy. Unpublished data indicate some decrease in artesunate efficacy [15
Recurrent parasitemia is known to occur when artemisinins are used as monotherapy for fewer than 5 days. This is not due to resistance but is thought to represent inadequate dosing of the drug, which has a very short half-life, because the recurrent parasites from these patients do not demonstrate decreased in vitro drug susceptibility. High initial parasite burden is strongly associated with recrudescent parasitemia after a 3-day course of artesunate [16
]. This association points to the sensitivity of treatment outcomes to the unique pharmacokinetic and pharmacodynamic factors of the artemisinins.
The emphasis on reporting clinical outcomes of only recrudescent failures and the removal of study subjects with new infections from the efficacy estimates also may complicate interpretation of efficacy studies [17•
]. Combinations that have high efficacy but short duration of action may give the impression of being the best drug choice. However, in areas of high transmission, short-acting combinations do not provide a prolonged period of prophylaxis after treatment, so that another illness episode can occur soon after the initial one. Thus, such combinations may not adequately decrease the malaria burden in areas of high transmission even when the reported efficacy is high.