Challenges regarding analysis of unbound fraction of highly bound protein antiretroviral drugs in several biological matrices: lack of harmonisation and guidelines

Development and validation of equilibrium dialysis UHPLC-MS/MS measurement procedures for total and unbound concentrations of bictegravir, dolutegravir, darunavir and doravirine in human plasma. Application to patients with HIV

BACKGROUND

Fixed-dose combinations of antiretroviral drugs are commonly used to treat HIV infection and therapeutic monitoring is not part of routine clinical practice. However, drug concentrations monitoring might have role in different clinical scenarios as well as for research purposes. This study aimed to develop and validate UHPLC-MS/MS procedures for measuring total and unbound concentrations of bictegravir, dolutegravir, darunavir and doravirine in human plasma.

MATERIAL AND METHODS

Equilibrium dialysis preceded sample preparation (based on protein precipitation) for measuring unbound antiretroviral concentrations. Chromatographic separations were achieved on an Acquity®-UPLC® HSS™-T3 column (50 mm × 2.1 mm; 1.8 µm) using a non-linear water/acetonitrile gradient containing 0.1 % formic acid at a 0.5 mL/min flow rate. Antiretrovirals were detected by tandem mass spectrometry in positive electrospray ionisation and multiple reaction monitoring modes.

RESULTS

No significant interferences or carry-over were observed. Imprecisions, absolute relative biases, normalised matrix effects and recoveries were ≤15.0 %, ≤11.1 %, (94.7-104.1)% and (96.7-105.5)%, respectively. Non-linear measuring intervals were observed between (25-10,000) µg/L for total/plasma dialysate concentrations and linearity schemes (1.00-100) µg/L for buffer dialysate concentrations.

CONCLUSIONS

The UHPLC-MS/MS procedures developed could be used for research purposes and therapeutic drug monitoring of antiretrovirals in routine clinical practice.

Measurement of free fraction, total concentration and protein binding for testosterone, triiodothyronine and thyroxine

Aims: Measuring the total and free concentrations of hormones is useful, but the technology to do this simultaneously is lacking. Methods: A new method offers the ability to measure these parameters concurrently for testosterone, thyroxine and triiodothyronine. Results: The free concentrations showed significant correlations with patients' vital statistics. Overall, 67% of correlations for total concentration showed that the new and classical methods had equal accuracy, or that comprehensive ultrafiltration was more accurate. The protein binding term was found to correlate significantly with the patients' luteinizing hormone, prostate-specific antigen and height. Conclusion: Comprehensive ultrafiltration for measuring the total concentration, free concentration and protein binding term uses less sample and is much faster than measuring these parameters with three separate methods.

Rapid equilibrium dialysis, ultrafiltration or ultracentrifugation? Evaluation of methods to quantify the unbound fraction of substances in plasma

In pharmacokinetics plasma protein binding (PPB) is a well-established parameter impacting drug disposition. The unbound fraction (f_u) is arguably regarded the effective concentration at the target site. Pharmacology and toxicology, increasingly use in vitro models. The translation of in vitro concentrations to in vivo doses can be supported by toxicokinetic modelling, e.g. physiologically based toxicokinetic models (PBTK). PPB of a test substance is an input parameter for PBTK. We compared three methods to quantify f_u: rapid equilibrium dialysis (RED), ultrafiltration (UF) and ultracentrifugation (UC) using twelve substances covering a wide range of Log P_ow (-0.1 to 6.8) and molecular weights (151 and 531 g/mol): Acetaminophen, Bisphenol A, Caffeine, Colchicine, Fenarimol, Flutamide, Genistein, Ketoconazole, α-Methyltestosterone, Tamoxifen, Trenbolone and Warfarin. After RED and UF separation, three polar substances (Log P_ow < 2) were largely unbound (f_u > 70%), while more lipophilic substances were largely bound (f_u < 33%). Compared to RED or UF, UC resulted in a generally higher f_u of lipophilic substances. f_u obtained after RED and UF were more consistent with published data. For half of the substances, UC resulted in f_u higher than the reference data. UF, RED and both UF and UC, resulted in lower f_u of Flutamide, Ketoconazole and Colchicine, respectively. For f_u quantifications, the separation method should be selected according to the test substance's properties. Based on our data, RED is suitable for a broader range of substances while UC and UF are suitable for polar substances.

Determination of unbound piperaquine in human plasma by ultra-high performance liquid chromatography tandem mass spectrometry

Piperaquine (PQ) is an antimalarial drug that is highly protein-bound. Variation in plasma protein contents may affect the pharmacokinetic (PK) exposure of unbound drug, leading to alteration of clinical outcomes. All published methods for determination of PQ in human plasma measure the total PQ including both bound and unbound PQ to plasma proteins. There is no published method for unbound PQ determination. Here we report an ultra-high performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) method for determination of PQ in human plasma filtrate prepared by filtering human plasma through Millipore Microcon® centrifugal filters (10k NMWL). The filter cup had to be treated with 5% benzalkonium chloride to reduce non-specific binding to the filter devices before filtration of plasma samples. Multiple reactions monitoring (MRM) of the ion pairs m/z 535/288 for PQ and m/z 541/294 for the internal standard (IS) was selected for quantification. When electrospray ionization (ESI⁺) was used, paradoxical matrix effect was observed despite the structure similarity of the deuterated IS: Ion suppression for PQ versus ion enhancement for the PQ-d₆, even though they were closely eluted: 0.62 min versus 0.61 min. Separation was achieved on Evo C₁₈ column (50 × 2.1 mm, 1.7 μm, Phenomenex Inc.) eluted with 10 mM NH₄OH and MeCN. When atmospheric pressure chemical ionization in positive mode (APCI⁺) was used for ion source, matrix effect diminished. Separation was achieved on a PFP column (30 × 2.1 mm, 1.7 μm, Waters, Corp.) eluted with aqueous 20 mM ammonium formate 0.14% trifluoroacetic acid (A) and methanol-acetonitrile (4:1, v/v) containing 0.1% trifluoroacetic acid (B) at 0.8 mL/min flow rate in a gradient mode: 30–30–80–80–30–30%B (0–0.1–1.0–1.40–1.41–1.50 min). The retention time was 0.67 min for both PQ and the IS. The method was validated with a linear calibration range from 20 to 5,000 pg/mL and applied to clinical samples.

A rapid and simple electrochemical detection of the free drug concentration in human serum using boron-doped diamond electrodes

Monitoring drug concentration in blood and reflecting this in the dosage are crucial for safe and effective drug treatment.

The Effect of Plasma Protein Binding on the Therapeutic Monitoring of Antiseizure Medications

Epilepsy is a widely diffused neurological disorder including a heterogeneous range of syndromes with different aetiology, severity and prognosis. Pharmacological treatments are based on the use, either in mono- or in polytherapy, of antiseizure medications (ASMs), which act at different synaptic levels, generally modifying the excitatory and/or inhibitory response through different action mechanisms. To reduce the risk of adverse effects and drug interactions, ASMs levels should be closely evaluated in biological fluids performing an appropriate Therapeutic Drug Monitoring (TDM). However, many decisions in TDM are based on the determination of the total drug concentration although measurement of the free fraction, which is not bound to plasma proteins, is becoming of ever-increasing importance since it correlates better with pharmacological and toxicological effects. Aim of this work has been to review methodological aspects concerning the evaluation of the free plasmatic fraction of some ASMs, focusing on the effect and the clinical significance that drug-protein binding has in the case of widely used drugs such as valproic acid, phenytoin, perampanel and carbamazepine. Although several validated methodologies are currently available which are effective in separating and quantifying the different forms of a drug, prospective validation studies are undoubtedly needed to better correlate, in real-world clinical contexts, pharmacokinetic monitoring to clinical outcomes.

Development of a simple and rapid method to determine the unbound fraction of dolutegravir, raltegravir and darunavir in human plasma using ultrafiltration and LC-MS/MS

Dolutegravir, raltegravir and darunavir are three antiretroviral drugs widely used in combined antiretroviral therapies. These three drugs are highly bound to plasma proteins. Compared to the total concentration, the concentration of unbound drug which is considered as the only pharmacological active form should be more informative to improve therapeutic drug monitoring in patients to avoid virological failure or toxicity. The aim of the present study was to develop an ultrafiltration protocol and a LC-MS/MS method to simultaneously determine the concentrations of the unbound dolutegravir, raltegravir and darunavir in human plasma. Finally, 150 μL of plasma was ultrafiltrated using Centrifree® ultrafiltration devices with ultracel YM-T membrane (cutoff 30 KDa) during 5 min at 37 °C at 1500 g. Then, 20 μL of the ultrafiltrate were injected into the LC-MS/MS system. The chromatographic separation was carried out on a BEH C18 column using a mobile phase containing deionized water and acetonitrile, both with 0.05 % (v/v) of formic acid, with a gradient elution at a flow rate of 0.5 mL/min. The run time was only 4 min. The calibration curve ranged from 0.5-200 ng/mL for dolutegravir, 1 to 400 ng/mL for raltegravir and 10-4000 ng/mL for darunavir. This method was validated with a good precision (inter- and intra-day CV% lower than 14 %) and a good accuracy (inter- and intra-day bias between -5.6-8.8 %) for all the analytes. This method is simple, reliable and suitable for pharmacokinetic studies.

Enabling direct and definitive free fraction determination for highly-bound compounds in protein binding assay

Protein binding determination for highly-bound compounds using equilibrium dialysis remains a challenge in drug discovery. The reasons are mainly three-fold; 1. due to their slow diffusion rate, highly-bound compounds require a much longer incubation time to reach dialysis equilibrium than typically needed; 2. highly-bound compounds are often hydrophobic and prone to non-specific binding in dialysis; 3. free drug concentration in the post incubation dialysate is too low for reliable analytical quantification. Modified equilibrium dialysis approaches include using diluted plasma for dialysis, or pre-saturating the non-specific binding sites in the dialysis device with compounds of interest prior to dialysis. In this study, we developed a customized equilibrium dialysis assay with an extended incubation time of 24 h, followed by microflow (μF) LC-MS/MS for bioanalysis, for direct and definitive free fraction determination of highly protein-bound compounds. The extended incubation time ensured the dialysis to reach equilibrium and saturating the non-specific binding sites, while μFLC-MS/MS provided far better sensitivity than the conventional LC-MS/MS typically used for post incubation bioanalysis. For a group of commercially available, highly protein-bound compounds, the free fraction data generated by the developed assay correlated very well with the literature values generated with diluted plasma method or pre-saturation method. This novel assay approach has been successfully used to generate protein binding results for highly-bound compounds to support ongoing drug discovery research.

Comparing ultrafiltration and equilibrium dialysis to measure unbound plasma dolutegravir concentrations based on a design of experiment approach

Dolutegravir therapeutic drug monitoring (TDM) could be improved by measuring the unbound dolutegravir plasma concentration (Cu), particularly in patients experiencing virological failure or toxicity despite achieving appropriate DTG total plasma concentrations. Equilibrium dialysis (ED) is the gold standard to measure Cu, but ED is time consuming, precluding its use in clinical practice. In contrast, ultrafiltration is applicable to TDM, but is sensitive to numerous analytical conditions. In order to evaluate measurements of Cu by ultrafiltration, ultrafiltration conditions were validated by comparison with ED. DTG concentrations were measured by LC–MS/MS. Three ultrafiltration factors (temperature, duration and relative centrifugal force [RCF]) were evaluated and compared to ED (25/37 °C), using a design of experiment strategy. Temperature was found to influence Cu results by ED (p = 0.036) and UF (p = 0.002) when results were analysed with ANOVA. Relative centrifugal force (2000 g) and time (20 min) interacted to influence Cu (p = 0.006), while individually they did not influence Cu (p = 0.88 and p = 0.42 for RCF and time). Ultrafiltration conditions which yielded the most comparable results to ED were 37 °C, 1000 g for 20 min. Ultrafiltration results greatly depended on analytical conditions, confirming the need to validate the method by comparison with ED in order to correctly interpret DTG Cu.

Pretreatment and non-specific binding in ultrafiltration device: Impact on protease inhibitor quantification

INTRODUCTION

Ultrafiltration (UF) is used to separate unbound drugs; however, non-specific binding (NSB) may be a limiting factor of this technique. Pretreatment of UF devices has been suggested to reduce NSB. Therefore, the pretreatment methodologies for UF devices were evaluated in order to test their effectiveness in reducing NSB of protease inhibitors (PIs).

METHODOLOGY

Two PIs (lopinavir-LPV and ritonavir-RTV) were tested. UF devices were pretreated with ultrapure water, Tween-20 or Tween-80. To evaluate the NSB, after UF devices being pretreated, ultrafiltrate solutions containing the analytes at two concentrations (low and high) were used. Samples were quantified by LC-MS/MS.

RESULTS

UF devices pretreated with Tween-5% had the lowest NSB for both analytes. NSB values varied between 7 and 11% at low concentration 16-34% at high LPV concentration, respectively. For RTV, NSB was approximately 6% for low concentration and 18% for high concentration. Failure to completely remove Tween in UF devices could results in an overestimation of NSB.

CONCLUSION

Pretreatment of UF device with Tween and subsequent removal proved to be effective in reducing NSB of PI.

Measurement of total and unbound bictegravir concentrations in plasma and cerebrospinal fluid by UHPLC-MS/MS

Bictegravir is a novel integrase strand transfer inhibitor, administrated in co-formulation with tenofovir alafenamide and emtricitabine (Biktarvy®), indicated in the management of HIV-1 infection in patients not previously treated with antiretroviral therapy. Bictegravir is highly bound to plasma proteins, and this significantly determines its clearance, solubility, and activity. These characteristics are crucial determinants of bictegravir penetration into human body compartments, as the central nervous system. We developed and validated UHPLC-MS/MS procedures to measure total and unbound bictegravir concentrations in plasma and cerebrospinal fluid. Simple protein precipitation with acetonitrile was implemented to prepare plasma and cerebrospinal fluid samples. Sample preparation was preceded by ultrafiltration for measuring unbound bictegravir concentrations. Chromatographic separations were achieved on an Acquity® UHPLC® BEH^TM (2.1 × 100 mm id, 1.7 μm) reverse-phase C₁₈ column using an isocratic mobile phase 20:80 (v/v) water/acetonitrile with 0.1% formic. Bictegravir and its internal standard (bictegravir-¹⁵N d₂) were detected by electrospray ionization mass spectrometry in positive and multiple reaction monitoring modes, using transitions of 450.2→289.2/145.4 and 453.2→289.2, respectively. Ultrafiltration procedures presented non-specific bindings of (8.6 ± 1.2) % for bictegravir in plasma and (26.6 ± 3.1) % for bictegravir in cerebrospinal fluid. Linearity was observed between (10.70-8560) μg/L, (1.07-856.0) μg/L for total and unbound bictegravir in plasma, and 0.107-26.75 μg/L for total and unbound bictegravir in cerebrospinal fluid. Imprecisions, absolute relative biases, normalized-matrix factors, and normalized-recoveries were ≤14.4%, ≤13.8%, (97.4-102.5) %, and (99.8-105.1) %, respectively. No significant interferences and carry-over were observed. The validated UHPLC-MS/MS procedures could be useful for pharmacokinetic and pharmacodynamic studies.

Posaconazole trough concentrations are not influenced by inflammation: A prospective study

During inflammation, several cytochrome P450 enzymes are downregulated. Recently it was shown that voriconazole metabolism is reduced during inflammation. Posaconazole, another triazole with broad-spectrum antifungal activity, is metabolised only to a limited extent by cytochrome P450 enzymes and to a wider extent by phase 2 enzyme systems. The aim of this study was to investigate posaconazole concentrations during inflammation. Patients aged ≥18 years receiving posaconazole prophylaxis or treatment for fungal infections were enrolled in a prospective observational study. Samples for posaconazole and C-reactive protein (CRP) concentrations were collected routinely for each patient. Longitudinal data analysis was performed to analyse the correlation between posaconazole serum trough concentrations and CRP values, corrected for potential factors that could influence the posaconazole concentration. Between August 2015 and June 2017, 64 patients were recruited to this study. Data for 55 patients (511 posaconazole samples) were included in the final analysis. The overall median posaconazole concentration was 1.8 mg/L [interquartile range (IQR) 1-2.9 mg/L, range 0.1-7.94 mg/L] and the overall median CRP concentration was 23.5 mg/L (IQR 5-75 mg/L, range 0-457 mg/L). Longitudinal data analysis showed that only the posaconazole daily dose (in mg/kg body weight) had a significant influence on posaconazole concentration after correction for other factors (P < 0.0001). Posaconazole concentrations were not influenced by CRP concentrations (P = 0.77). Posaconazole concentrations are not influenced by inflammation, reflected by CRP concentration. Therefore, more frequent therapeutic drug monitoring of posaconazole during inflammation or after an infection subsides is not necessary.

Chromatographic methods in HIV medicine: Application to therapeutic drug monitoring

HIV antiretroviral therapy spans several different drug classes, meant to combat various aspects of viral infection and replication. Many authors have argued the benefits of therapeutic drug monitoring (TDM) for the HIV patient including compliance assurance and assessment of appropriate drug concentrations; however, the array of drug chemistries and combinations makes TDM an arduous task. HPLC-UV and LC-MS/MS are both frequent instruments for the quantification of HIV drugs in biological matrices with investigators striving to balance sensitivity and affordability. Plasma, the dominant matrix for these analyses, is prepared using protein precipitation, liquid-liquid extraction or solid-phase extraction depending on the specific complement of analytes. Despite the range of polarities found in drug classes relevant to HIV therapeutics, most chromatographic separations utilize a hydrophobic column (C₁₈ ). Additionally, as the clinically relevant samples for these assays are infected with HIV, along with possible co-infections, another important aspect of sample preparation concerns viral inactivation. Although not routine in clinical practice, many published analytical methods from the previous two decades have demonstrated the ability to conduct TDM in HIV patients receiving various medicinal combinations. This review summarizes the analytical methods relevant to TDM of HIV drugs, while highlighting respective challenges.

Determination of dolutegravir's unbound fraction in human plasma using validated equilibrium dialysis and LC-MS/MS methods

Assessment of the unbound pharmacologically active fraction (fu; as the ratio of unbound to total concentration) of dolutegravir could improve therapeutic drug monitoring (TDM) in patients that experience virological failure or toxicity, despite receiving adequate total concentrations. This study evaluated (i) dolutegravir's fu through equilibrium dialysis (ED), (ii) the pre-analytical parameters that influence fu, and (iii) fu's inter-individual variability in HIV patients. Validation of the LC-MS/MS method followed FDA guidelines. The results, based on coefficients of variation (results from nominal concentrations <15%), allowed accurate measurement of unbound and total dolutegravir concentrations. Equilibrium during ED was obtained in 4h. Sparse non-specific binding (9%) was observed, allowing results interpretation without interference. Steps before analysis (e.g., conservation at +4°C, freeze/thaw cycles) did not influence fu, allowing easy integration of fu analysis within laboratory routines. Anticoagulants from samples (citrated versus heparinized; p<0.001) and hemolysis (p=0.007) influenced fu and could lead to misinterpretation. Developed was then performed to the HIV-patients' plasma (n=54). Results, expressed as median InterQuartile Range [25%;75%] were 0.45% IQR [0.38; 0.55] for fu, 9.26μg/L IQR [4.62; 15.14] for unbound, and 2035μg/L IQR [878.5; 2640] for total concentration. The high inter-individual variability observed in the unbound form from HIV patients was a first step towards integrating dolutegravir TDM.

A new application of micellar liquid chromatography in the determination of free ampicillin concentration in the drug-human serum albumin standard solution in comparison with the adsorption method

The determination of free drug concentration is a very important issue in the field of pharmacology because only the unbound drug fraction can achieve a pharmacological effect. Due to the ability to solubilize many different compounds in micellar aggregates, micellar liquid chromatography (MLC) can be used for direct determination of free drug concentration. Proteins are not retained on the stationary phase probably due to the formation of protein - surfactant complexes which are excluded from the pores of stationary phase. The micellar method is simple and fast. It does not require any pre-preparation of the tested samples for analysis. The main aim of this paper is to demonstrate a completely new applicability of the analytical use of MLC concerning the determination of free drug concentration in the standard solution of human serum albumin. The well-known adsorption method using RP-HPLC and the spectrophotometric technique was applied as the reference method. The results show that the free drug concentration value obtained in the MLC system (based on the RP-8 stationary phase and CTAB) is similar to that obtained by the adsorption method: both RP-HPLC (95.83μgmL(-1), 79.86% of free form) and spectrophotometry (95.71μgmL(-1), 79.76%). In the MLC the free drug concentration was 93.98μgmL(-1) (78.3%). This indicates that the obtained results are within the analytical range of % of free ampicillin fraction and the MLC with direct sample injection can be treated like a promising method for the determination of free drug concentration.