In this study, we have been able to describe detailed systemic and intracellular drug-drug interactions between TDF, ABC, and LPV/r.
These data confirm previous reports showing the lack of interaction between TDF, 3TC, and ABC at the level of plasma (11
), as well as the lack of interaction at the intracellular level in vitro (38
) and in vivo (15
Interestingly, the median intracellular half-life of elimination of TFV-DP found in this study, 150 h, provides confirmation of our previous estimate of 180 h (35
) and is very close to the value reported by Hawkins et al. (15
), around 150 h.
By comparing the LPV/r and NVP groups, we have observed a decrease in ABC exposure (40%) when ABC was combined with LPV/r similar to that recently published by Waters et al. (50
), who reported a 32% decrease in plasma ABC exposure following the addition of LPV/r for HIV-infected patients on ABC (600 mg once daily). The mechanism of induction of ABC glucuronidation by ritonavir and/or LPV proposed by Waters et al. on the basis of their own work and previous work (10
) seems to be well founded. To our knowledge, PK data for intracellular CBV-TP in the presence and absence of LPV/r were not reported. While a lower plasma ABC concentration was observed in this group receiving ABC-LPV/r, our observation of a reverse trend for CBV-TP at the PBMC level was very surprising and certainly deserves confirmation. Globally, as expected, TFV-DP, 3TC-TP, and CBV-TP presented flat PK profiles from 0 to 4 h. However, for a few patients of the LPV/r group, the CBV-TP profile fluctuated more than usual. It seems likely that, for these patients, the intracellular CBV-TP concentration at the 4-h time point was near the Cmax
but was not included in the 0- to 4-h period. Consequently, for CBV-TP, AUC0-4
was not a representative part of the total AUC and was possibly underestimated. This does not challenge our findings but rather explains why only a trend was observed for increased intracellular CBV-TP exposure in the presence of LPV/r. Nevertheless, this observation is consistent with the efficacy of the combination containing ABC plus LPV/r in comparison with other nucleoside combinations, as seen in the present study and in the KLEAN study (9
In contrast to ABC, we may observe an increase of around 50% in plasma TFV exposure when TDF is administered with LPV/r, confirming the systemic interaction between TDF and LPV/r already observed in a previous study (22
) conducted with healthy volunteers. In the present study, conducted with chronically treated HIV-infected patients, the effect was a bit more pronounced. But these results in this study cannot be directly compared with those of the crossover drug interaction study with healthy volunteers, and this weak gap could be explained either by differences in the two populations studied (healthy volunteers versus chronically treated patients, age, gender structure, sample size, and diet) or by the way PK parameters were estimated. Nevertheless, regarding our evaluation of PK parameters (over only 4 h instead of 24 h), we may note that the mean Ctrough
with or without LPV/r was 91.8 or 60.8 ng/ml, respectively, which compares well with that reported by Kearney et al., 99.8 or 63.8 ng/ml, respectively. This shows, furthermore, that Ctrough
is not the parameter involved in the difference in the AUC increase. Since Ctrough
is the same in both studies, and considering that the Ctrough
could also be used as the 24-h concentration, because after the last sampling time (4 h) we are in the terminal phase of TFV elimination, as shown by Kearney et al., we may evaluate the AUC0-24
for TFV. By doing so, we obtain a mean TFV AUC0-24
(without LPV/r) of 2,904 ng·h/ml, a value very close to that for the group of healthy volunteers without LPV/r, reported at 2,870 ng·h/ml. Thus, the use of AUC0-4
for group comparison was suitable here. It is worth noting that our results and those of Kearney et al. contradict recently published data (25
), where no statistically significant difference was found between PK parameters for HIV-infected patients receiving TDF with and without LPV/r. Since the patient demographic structure (age and gender) was very close to that of our study, the reason for this discrepancy remains unclear to date.
Interestingly, the original contribution of this study is to show that the TFV increase observed at the plasma level is accompanied by a TFV-DP increase of a similar magnitude at the intracellular level. TFV-DP exposure was increased by 59% when LPV/r was coadministered. This confirms the existence of an interaction and demonstrates a direct correlation between plasma TFV levels and intracellular TFV-DP concentrations.
As far as the mechanism of the TDF-LPV/r interaction is concerned, several hypotheses that involve intestinal or renal function have been proposed. In fact, PIs are known to interact with efflux transporters such as P-glycoprotein (Pgp) and multidrug resistance-associated proteins (7
). LPV and ritonavir could thus affect TDF absorption by inhibiting Pgp activity, as previously observed in in vitro experiments (43
). Other investigators recently reported on a possible renal implication (40
), but some of these results are contradictory, and so far there is no clear demonstration of involvement of kidney function in this interference. Nevertheless, in parallel with this increase in exposure, patients had not developed overt adverse effects such as nephrotoxicity (20
Another original contribution of this paper is the observation that intracellular TFV-DP exposure is significantly higher for women than for men, even if we must remain cautious owing to the low number of women in the patient groups and the P
value of 0.045. A similar trend was observed for 3TC-TP but was not statistically significant, very likely due to the lower number of patients receiving 3TC than TDF. Sex-related differences in intracellular concentration for NRTI-TP metabolites have already been reported for zidovudine (1
), 3TC (1
), and ABC (13
); such differences are shown here for TFV-DP and have to be confirmed by other studies. This significant difference was seen only for the triphosphate metabolite at the intracellular level, not for TFV at the plasma level, suggesting a difference in the capacities of men and women to phosphorylate NRTIs and N(t)RTIs, whether directly related to sex hormones or not.
Finally, concerning the possible inhibition of PNP by TDF phosphate metabolites and its possible consequence on an increase in the dGTP pool, no difference was seen in dGTP levels between patients receiving or not receiving TDF. These in vivo results are in agreement with previous in vitro observations showing similar dGTP levels in uninfected human CD4+
T cells treated with ABC, TDF, TDF-ABC, and TDF-ABC-3TC (41
) and revealing no effect of TDF alone or with other NRTIs on dNTP pools (46
). Of note, TFV-DP was not entirely washed out after 4 weeks in patients of groups 1 and 3. Still, considering the half-life and the washout period, the remaining TFV-DP level was around only 4.5% of the initial TFV-DP level. In addition, TFV monophosphate (TFV-MP) is known to be the main inhibitor of PNP (39
). Because there is no enzymatic limiting step in the TFV phosphorylation cascade, TFV-MP levels cannot be higher than those of TVF-DP; the former are approximately fivefold lower than the latter (8
). Thus, taking into account the TFV-DP concentration after the washout period, the remaining TFV-MP level would be markedly lower than its Ki
for PNP inhibition (39
). Therefore, under these conditions, we can reasonably consider that the comparison of dGTP levels as a marker of PNP inhibition (before and after TDF washout) is relevant.
Some limitations of this study have to be pointed out. First of all, the sample sizes of the groups included in this study were small and do not allow high-powered statistical tests. The study design also leads to limitations. For example, the potential impact of TDF on 3TC or ABC was directly investigated with the same patients receiving TDF and after TDF discontinuation, whereas the influence of 3TC or ABC on TDF was indirectly observed by comparison of TDF PK parameters in the two combinations.
Nevertheless, this work clearly shows the interest of addressing both systemic and intracellular metabolism for the understanding of N(t)RTI- and NRTI-based anti-HIV therapies.