Ultrasensitive method to quantify intracellular zidovudine mono-, di- and triphosphate concentrations in peripheral blood mononuclear cells by liquid chromatography-tandem mass spectrometry

The determination of human peripheral blood mononuclear cell counts using a genomic DNA standard and application in tenofovir diphosphate quantitation

A fluorescent quantitation method to determine PBMC-derived DNA amounts using purified human genomic DNA (gDNA) as the reference standard was developed and validated. gDNA was measured in a fluorescence-based assay using a DNA intercalant, SYBR green. The fluorescence signal was proportional to the amount (mass) of DNA in the sample. The results confirmed a linear fit from 0.0665 to 1.17 μg/μL for gDNA, corresponding to 2.0 × 10⁶ to 35.0 × 10⁶ cells/PBMC sample. Intra-batch and inter-batch accuracy (%RE) was within ±15%, and precision (%CV) was <15%. Benchtop stability, freeze/thaw stability and long term storage stability of gDNA in QC sample matrix, PBMC pellets samples, and pellet debris samples, respectively, as well as dilution linearity had been established. Consistency between hemocytometry cell counting method and gDNA-based counting method was established. 6 out of 6 evaluated PBMC lots had hemocytometry cell counts that were within ±20% of the cell counts determined by the gDNA method. This method was used in conjunction with a validated LC-MS/MS method to determine the level of tenofovir diphosphate (TFV-DP), the active intracellular metabolite of the prodrugs tenofovir alafenamide (TAF) and tenofovir disoproxil fumarate (TDF), measured in PBMCs in clinical trials of TAF or TDF-containing fixed dose combinations.

Direct and indirect quantification of phosphate metabolites of nucleoside analogs in biological samples

Nucleoside reverse transcriptase inhibitors (NRTIs) are prodrugs that require intracellular phosphorylation to active triphosphate nucleotide metabolites (NMs) for their pharmacological activity. However, monitoring these pharmacologically active NMs is challenging due to their instability, high hydrophilicity, and their low concentrations in blood and tissues. Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) is the gold standard technique for the quantification of NRTIs and their phosphorylated NMs. In this review, an overview of the publications describing the quantitative analysis of intracellular and total tissue concentration of NMs is presented. The focus of this review is the comparison of the different approaches and challenges associated with sample collection, tissue homogenization, cell lysis, cell counting, analyte extraction, sample storage conditions, and LC-MS analysis. Quantification methods of NMs via LC-MS can be categorized into direct and indirect methods. In the direct LC-MS methods, chromatographic retention of the NMs is accomplished by ion-exchange (IEX), ion-pairing (IP), hydrophilic interaction (HILIC), porous graphitic carbon (PGC) chromatography, or capillary electrophoresis (CE). In indirect methods, parent nucleosides are 1st generated from the dephosphorylation of NMs during sample preparation and are then quantified by reverse phase LC-MS as surrogates for their corresponding NMs. Both approaches have advantages and disadvantages associated with them, which are discussed in this review.

Toward highly sensitive and reproducible LC-MS/MS analysis of MK-8591 phosphorylated anabolites in human peripheral blood mononuclear cells

Aim: MK-8591 (EFdA), a novel anti-HIV nucleoside analog, is converted to mono-, di- and tri-phosphates (MK-8591-MP, MK-8591-DP and MK-8591-TP) intracellularly, among which MK-8591-TP is the active pharmacological form. An ultrasensitive LC-MS/MS assay was required to measure MK-8591-DP and MK-8591-TP levels in human peripheral blood mononuclear cells (PBMCs). Sensitivity and reproducibility were major bottlenecks in these analyses. Materials and methods: Human PBMCs were isolated from blood and lysed with 70/30 methanol/RPMI-1640. An LC-MS/MS method was developed to simultaneously quantify MK-8591-DP and MK-8581-TP in PBMC lysates. Results: Low flow LC and dimethyl sulfoxide mediated signal enhancement enabled an extreme sensitivity with limit of quantitation at 0.1 ng/ml. Assay accuracy was 92.5-106% and precision was 0.7-12.1% for a linear curve range of 0.1-40 ng/ml. Matrix variability and interference liability were comprehensively evaluated. Conclusion: Our study findings and steps taken in addressing clinical sample issues help understand and overcome the challenges facing intracellular nucleotide analog analysis.

High Plasma Concentrations of Zidovudine (AZT) Do Not Parallel Intracellular Concentrations of AZT-Triphosphates in Infants During Prevention of Mother-to-Child HIV-1 Transmission

OBJECTIVES

Zidovudine (AZT) is mainly used to prevent mother-to-child HIV-1 transmission (PMTCT). Despite serious concerns on AZT-associated toxicity, there is little information on pharmacokinetics of intracellular AZT metabolites in infants.

METHODS

We conducted a prospective study in 31 HIV-uninfected infants who received AZT for PMTCT. Blood samples were obtained from 14 infants on postdelivery days (PDD) 1, 7, 14, and 28 and from 17 infants at 0 and 4 hours after dosing on PDD-1. Plasma AZT concentrations (pAZT) and intracellular concentrations of AZT-monophosphate (icAZT-MP), diphosphate (icAZT-DP), and triphosphate (icAZT-TP) were determined.

RESULTS

Plasma AZT and icAZT-MP concentrations were 2713 nmol/L and 79 fmol/10 cells in PDD-1, but decreased to 1437 nmol/L and 31 fmol/10 cells by PDD-28 (P = 0.02 and P = 0.07 for all PDDs, respectively), whereas those of icAZT-DP and icAZT-TP remained low throughout the sampling period (P = 0.29 and P = 0.61 for all PDDs, respectively) There were no differences in icAZT-TP between infants of the 2 mg/kg 4 times a day dose and 4 mg/kg twice daily dose (P = 0.25), whereas pAZT and icAZT-MP levels were higher in the latter (P < 0.01 and <0.01, respectively). The pAZT and icAZT-MP significantly increased from 0 to 4 hours after dosing (P < 0.001 and <0.001, respectively), whereas icAZT-DP, icAZT-TP levels were not changed (P = 0.41 and 0.33, respectively).

CONCLUSIONS

The level of icAZT-TP did not change with age, time, or a single dose despite the wide range of pAZT concentration. A safer dosage needs to be determined because high pAZT levels do not parallel those of icAZT-TP.

Enhancement of Polymerase Activity of the Large Fragment in DNA Polymerase I from Geobacillus stearothermophilus by Site-Directed Mutagenesis at the Active Site

The large fragment of DNA polymerase I from Geobacillus stearothermophilus GIM1.543 (Bst DNA polymerase) with 5'-3' DNA polymerase activity while in absence of 5'-3' exonuclease activity possesses high thermal stability and polymerase activity. Bst DNA polymerase was employed in isothermal multiple self-matching initiated amplification (IMSA) which amplified the interest sequence with high selectivity and was widely applied in the rapid detection of human epidemic diseases. However, the detailed information of commercial Bst DNA polymerase is unpublished and well protected by patents, which makes the high price of commercial kits. In this study, wild-type Bst DNA polymerase (WT) and substitution mutations for improving the efficiency of DNA polymerization were expressed and purified in E. coli. Site-directed substitutions of four conserved residues (Gly³¹⁰, Arg⁴¹², Lys⁴¹⁶, and Asp⁵⁴⁰) in the activity site of Bst DNA polymerase influenced efficiency of polymerizing dNTPs. The substitution of residue Gly³¹⁰ by alanine or leucine and residue Asp⁵⁴⁰ by glutamic acid increased the efficiency of polymerase activity. All mutants with higher polymerizing efficiency were employed to complete the rapid detection of EV71-associated hand, foot, and mouth disease (HFMD) by IMSA approach with relatively shorter period which is suitable for the primary diagnostics setting in rural and underdeveloped areas.