Emtricitabine and lamivudine concentrations in saliva: a simple suitable test for treatment adherence. Journal of Antimicrobial Chemotherapy
ART constitutes the cornerstone of HIV treatment and prevention. Successful ART results in suppressed viral load (VL) levels. Courlet et al. tested whether non-invasive saliva sampling could represent an alternative or clinically useful addition to VL measurements and genotype determination among patients suspected of non-adherence. If available as a point-of-care test, it would provide the clinicians with an objective assessment of recent adherence during the clinical visit.
The authors aimed to evaluate whether saliva would constitute a suitable matrix for ART adherence monitoring, by examining the correlation between emtricitabine and lamivudine levels simultaneously measured in plasma and saliva. 73 patients participating in the SHCS agreed to donate a non-stimulated saliva sample simultaneously to the blood sample collected as part of the biannual SHCS follow-up visit. Emtricitabine and lamivudine levels were quantified by an LC/MS-MS assay, using stable isotopically labelled internal standards also adapted for saliva concentration determination.
Overall, 47 and 26 paired plasma and saliva concentrations were collected for emtricitabine and lamivudine determination, respectively. In addition, six saliva samples were collected from three SHCS patients not receiving emtricitabine or lamivudine, and from three HIV-negative individuals, to evaluate the selectivity of the method.
Emtricitabine plasma and saliva concentrations ranged from 8 to 2’471 ng/mL and 23 to 1’017 ng/mL, respectively. The time between sampling and last drug intake (time after dose; TAD) ranged from 0.8 to 37.3 h. For lamivudine, plasma and saliva concentration varied between 45 and 3’183 ng/mL and between 15 and 718 ng/mL, respectively, with a TAD of 1.3 to 27 h. The results show that emtricitabine and lamivudine saliva concentrations are moderately correlated with plasma concentrations (correlation coefficient 0.63 and 0.58 for emtricitabine and lamivudine, respectively).
Median saliva/plasma ratios were 0.61 for emtricitabine and 0.35 for lamivudine, with high variability (coefficient of variation 89% and 101% for emtricitabine and lamivudine, respectively). This variability can be explained by multiple factors involving drug properties (i.e. protein binding, saliva pH, salivary flow rates and degree of ionization) or patient characteristics (i.e. renal clearance or adherence issues).
Importantly, saliva concentrations from individuals not receiving emtricitabine or lamivudine were below the limit of quantification (LOQ) in saliva (<10 ng/mL), confirming the specificity of the method. Assuming that emtricitabine in saliva follows the same kinetics as in plasma, the t½ of emtricitabine in saliva calculated for each patient using a population pharmacokinetics model of emtricitabine gave a mean t½ of 9.4 ± 0.88 h (minimum = 8.4 h, maximum = 12.1 h). It can be extrapolated that after 31 h (i.e. 3.3 × t½) emtricitabine levels in saliva would lie above the assay LOQ of 10 ng/mL in all patients, whereas after 60 h and 72 h (2.5 days and 3 days TAD, respectively) the saliva emtricitabine concentration would fall below the LOQ cut-off of the assay in 91% and 100% of patients, respectively. Thus, knowing the emtricitabine or lamivudine concentrations determined in saliva allows the identification of non-adherent patients who have missed their ART for at least the last 3 days.
In conclusion, despite a limited correlation between plasma and saliva concentrations, saliva constitutes a suitable non-invasive surrogate to identify patients who are non-adherent to ART. These first results have been obtained using an MS assay. However, if developed as a qualitative, immunoassay-based, easy-to-use and non-invasive point-of-care test, this assay would be of invaluable clinical usefulness to reduce the number of unnecessary VL measurements and genotype determinations in cases of proven non-adherence, and may thus prevent unnecessary switches to second-line ART.