Anatomical sanctuary sites in HIV-infected patients, where local drug exposure is lower than systemic compartment, are currently under intense investigation because they are suspected of hindering viral elimination by antiretroviral therapy (ART) and acting as sites for the selection of drug-resistant viruses during combination treatment. Especially the brain, the largest sanctuary site, in which residual viruses may cause chronic encephalitis and neurocognitive disorders, is one of the hottest foci of current HIV researches. Raltegravir, one of the preferred integrase inhibitors in the current ART guidelines, is highly effective in penetrating the central nervous system,1 although a high interpatient variability has also been reported.2,3
Anatomically, the blood–cerebrospinal fluid (CSF) barrier makes tight junction and consists of choroid plexus epithelial cells in the cerebral ventricle. The adenosine triphosphate–binding cassette transporter B1 (ABCB1), also known as P-glycoprotein or multidrug resistance protein 1, and the adenosine triphosphate–binding cassette transporter G2 (ABCG2), also known as breast cancer resistance protein, are expressed on the CSF side of choroid plexus epithelial cells, and both are involved in the active transport of drugs.4,5 Moreover, ABCB1 and ABCG2 are also expressed in the intestines and contribute to the absorption of the drugs. Recently, raltegravir was found to be a substrate of both ABCB1 and ABCG2.6 In the present study, we analyzed the relations between raltegravir plasma and CSF concentrations and single-nucleotide polymorphisms (SNPs) of ABCB1 and ABCG2 genomes.
MATERIALS AND METHODS
HIV-1–infected patients treated with raltegravir-containing regimens (raltegravir 400 mg twice daily with 2 nucleotide/nucleoside reverse transcriptase inhibitors and/or protease inhibitors) were recruited at the AIDS Clinical Center, National Center for Global Health and Medicine, Tokyo, Japan. Blood samples were withdrawn into heparinized tubes 12 hours after raltegravir dosing (trough level), and the plasma was separated and stored at −80°C. Stocked residues of CSF samples taken 3–4 hours after raltegravir dosing for clinical purposes were also subjected to analysis. The Ethics Committee for Human Genome Studies at the National Center for Global Health and Medicine approved this study (NCGM-A-000122-02) and allowed us the use of only residues of samples that were originally obtained for clinical purposes. Each patient provided a written informed consent.
Plasma and CSF raltegravir concentrations were measured by the reverse-phase high-performance liquid chromatography (HPLC) method. Briefly, 200 µL of plasma or CSF and 400 µL of ethyl acetate were vortexed in a tube for 10 seconds and centrifuged. The organic phase was transferred to a new tube and evaporated to dryness. Subsequently, the residue was reconstituted in 250 µL of mobile phase, and 50 µL was injected into HPLC. Chromatography was performed, using Chromaster HPLC system (Hitachi, Tokyo, Japan) with RF-10A fluorescence detector (Shimadzu, Kyoto, Japan). Inertsil ODS-3 column (150 × 4.6 mm, 5-μm particle size; GL Sciences, Tokyo, Japan) was used as the analytical column. The flow rate was maintained at 1.5 mL per minute with fluorescence detection at 307 nm (excitation) and 415 nm (emission). The mobile phase consisted of acetonitrile/ethanol/phosphoric acid/water (20.8:20.8:0.1:58.3, vol/vol). Raltegravir calibration standards ranged from 10 to 2500 ng/mL. The accuracy of the analysis at 3 concentration levels ranged from −8.4% to +4.9%. Intraassay and interassay precisions were <4.8% and <7.6%, respectively. This assay was validated for both plasma and CSF raltegravir concentrations.
Genomic DNA was isolated from peripheral blood mononuclear cell, using a QIAamp DNA Mini Kit (Qiagen, Hilden, Germany). Genotyping of allelic variants of ABCB1 1236 C>T (rs1128503), 2677 G>T/A (rs2032582), 3435 C>T (rs1045642), 4036 A>G (rs3842), and ABCG2 421 C>A (rs2231142) was carried out using the TaqMan Drug Metabolism Assays by the ABI PRISM 7900HT sequence detection system (Applied Biosystems, Foster City, CA), according to the protocol provided by the manufacturer.
Differences between the groups were analyzed for statistical significance using the Kruskal–Wallis test. P values <0.05 denoted the presence of statistically significant difference. Analysis was performed using the SPSS Statistics software version 21 (IBM, Armonk, NY).
Plasma samples were collected from 31 patients, and stocked CSF samples from another group of 14 patients were used for the measurement of raltegravir concentrations.
All 45 patients (Japanese = 44, Myanmarian = 1) were subjected to SNP analysis of ABCB1 and ABCG2 genomes (Table 1). At position 1236 of ABCB1 gene, CC, CT, and TT genotypes were identified in 7, 21, and 17 patients, respectively. At position 2677, GG, GT, TT, GA, TA, and AA genotypes were identified in 8, 14, 11, 7, 4, and 1 patients, respectively. At position 3435, CC, CT, and TT genotypes were identified in 14, 17, and 14 patients, respectively. At position 4036, AA, AG, and GG genotypes were identified in 25, 18, and 2 patients, respectively. None of the genotypes of these SNPs in ABCB1 genome showed significant correlation with raltegravir concentration in plasma or CSF. At position 421 of ABCG2 gene, CC, CA, and AA genotypes were identified in 26, 14, and 5 patients, respectively. There was no significant correlation between the genotype at position 421 and trough concentration of raltegravir in plasma (Fig. 1A). However, in all 3 AA genotype holders, CSF raltegravir concentration was less than the lower limit of quantification (10 ng/mL) (Fig. 1B). Furthermore, in one of 4 CA genotype holders, CSF raltegravir concentration was below the detection limit, although it was higher than 25 ng/mL in any of the 7 CC genotype holders. The CA and AA genotype holders had significantly lower raltegravir concentrations in the CSF than the CC genotype holders (P = 0.016), when the concentration below the lower limit of quantification was considered 10 ng/mL.
ABCG2 is diffusely expressed, whereas ABCB1 is weakly expressed on the CSF side of choroid plexus epithelial cells,7,8 suggesting that the contribution of ABCB1 may be minor and that ABCG2 expression level in the choroid plexus is more likely to influence raltegravir concentration in the CSF than ABCB1. Previous studies indicated that genetic polymorphism of ABCG2 altered the protein expression level in plasmid transfection experiments.9,10 Especially, C to A nucleotide substitution at position 421 significantly reduced the expression. The low expression induced by this nucleotide substitution may impair raltegravir transport from capillary blood to CSF, resulting in low raltegravir concentrations in CSF in holders of the CA/AA genotype at position 421. However, this SNP did not alter plasma raltegravir concentration significantly. Transporters other than ABCG2 may also exist in the intestines and further enhance raltegravir absorption. The presence of any antiretroviral at a concentration lower than that required for viral suppression could select drug-resistant HIV variants. In fact, we reported previously one patient with CSF raltegravir-resistant HIV variant, although the variant was not detected in the plasma.11 The present study indicate that the genotype of this patient was AA at position 421 and that raltegravir concentration was below the lower limit of quantification in the CSF of this patient. Special attention should be paid to the raltegravir-containing ART of individuals with the CA/AA genotype at position 421 with active viral replication in the CNS, such as patients with HIV encephalitis.
Our study has certain limitations. Raltegravir concentrations were measured in plasma at trough level in 31 patients, and it was measured in stocked CSF samples of another group of 14 patients. First, we could not investigate the correlation between plasma and CSF concentrations because no paired plasma and CSF samples from the same subjects were available. Second, the time of CSF sampling in relation to raltegravir dosing varied among 3–4 hours. However, the population pharmacokinetic modeling of raltegravir concentration in the CSF showed a stable time course regardless of the dosing time.2,12 Therefore, it is unlikely that the sampling time had a large impact on CSF concentration of the CA/AA genotype at position 421 in ABCG2 gene. Further analysis of the correlation between ABCG2 genotype and raltegravir CSF concentration is warranted.
The authors thank Dr. Fumihide Kanaya for helping to prepare the manuscript. They also thank the clinical and laboratory staff of the AIDS Clinical Center, National Center for Global Health and Medicine, for the helpful support.
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Keywords:© 2014 by Lippincott Williams & Wilkins
antiretroviral therapy; raltegravir; cerebrospinal fluid concentrations; blood–cerebrospinal fluid barrier; adenosine triphosphate–binding cassette transporter G2