A review of recent advances in microsampling techniques of biological fluids for therapeutic drug monitoring

Highlights

• Recent advances in microsampling techniques for TDM are illustrated.

• Classification of microsampling is based on type of biological matrices used.

• Advantages and shortcomings of each microsampling technique are discussed

• Latest applications of established miniaturized sampling techniques are presented.

Abstract

Conventional sampling of biological fluids often involves a bulk quantity of samples that are tedious to collect, deliver and process. Miniaturized sampling approaches have emerged as promising tools for sample collection due to numerous advantages such as minute sample size, patient friendliness and ease of shipment. This article reviews the applications and advances of microsampling techniques in therapeutic drug monitoring (TDM), covering the period January 2015 – August 2020. As whole blood is the gold standard sampling matrix for TDM, this article comprehensively highlights the most historical microsampling technique, the dried blood spot (DBS), and its development. Advanced developments of DBS, ranging from various automation DBS, paper spray mass spectrometry (PS-MS), 3D dried blood spheroids and volumetric absorptive paper disc (VAPD) and mini-disc (VAPDmini) are discussed. The volumetric absorptive microsampling (VAMS) approach, which overcomes the hematocrit effect associated with the DBS sample, has been employed in recent TDM. The sample collection and sample preparation details in DBS and VAMS are outlined and summarized. This review also delineates the involvement of other biological fluids (plasma, urine, breast milk and saliva) and their miniaturized dried matrix forms in TDM. Specific features and challenges of each microsampling technique are identified and comparison studies are reviewed.

Introduction

Conventional sampling of biological fluid, such as blood, plasma and urine, is a tedious and labor-intensive process. Generally, samples are collected in large volumes (> 1 mL) and further proceed to multi-stage sample preparation to obtain the cleanest samples for analysis [1]. Biological fluid samples that are collected in bulk volumes have tedious logistical requirements and their collection can be problematic for patients. For instance, in the current practice of blood sampling, blood samples are collected intravenously from the volar forearm of patients with a hypodermic needle. This venipuncture sampling technique is invasive for patients, especially to children and critically ill patients and necessitates a phlebotomist or trained medical personnel to perform it [1]. Thus, for patients that require routine diagnostic status monitoring and therapeutic drug monitoring (TDM), the frequency with which they visit medical premises for blood sampling has significantly increased. This phenomenon will potentially affect certain patients who are prone to phobias and are reluctant to receive appropriate therapies, indirectly leading to possible unexpected recurrences of disease and high mortality rates. Moreover, collected biological samples must be delivered to a central diagnostic laboratory for an accurate diagnosis. The delivery of samples to the designated laboratory is a time-consuming and costly process. Samples are delivered and stored under deep-freeze storage conditions (-20°C) to prevent sample degradation and extend their stability during transportation, resulting in an additional cost in shipping [2]. The turnaround time for diagnostic results varies from hours to days depending on the distance between the clinic and diagnostic laboratory. As the majority of the analytical and diagnostic services in centralized laboratories are available only in highly populated cities, the accessibility of biological samples from rural areas or remote health outposts is limited, which may subsequently cause a delay in disease diagnosis.

Microsampling is a minimally invasive technique for the collection of minute samples (< 100 µL) from the human body for analysis [3]. Over the past decades, several innovative miniaturized sampling methods have been developed mainly to overcome the drawbacks and challenges in conventional sampling for biological fluid. These sampling tools include dried blood spot (DBS), volumetric absorptive microsampling (VAMS) and dried matrix spot (DMS). Among these novel microsampling techniques, DBS can be regarded as the most widespread technique. Guthries and Susi [4] reported the application of DBS in newborn screening and diagnosis for phenylketonuria in 1963. Since then, DBS has been widely used for newborn screening [5], especially in developed countries. VAMS was introduced to overcome the drawbacks encountered by DBS, including uncertainty of blood volume in sub-punched DBS, hematocrit bias and spot inhomogeneity [6]. Generally, blood volume in fixed-size sub-punch DBS varies according to the hematocrit value and homogeneity of spotted blood while the VAMS technique allows the collection of a fixed volume of blood sample (approximately 10 µL) directly from a finger prick or pool of blood. Regardless of blood hematocrit, the collected dried blood sample is analyzed as a whole [7]. DMS, on the other hand, is a technique that involves the application of a small amount of biological fluids on paper substrates. Similar to DBS, biological fluids including plasma, urine, breast milk and saliva have been spotted on paper discs, resulting in dried plasma spot (DPS) [8], [9], [10], dried urine spot (DUS) [11,12], dried breast milk spot (DBMS) (also known as dried milk spot) [13,14], and dried saliva spot (DSS) [15] for various applications [16].

The implementation of microsampling techniques has been reported on a wide range of applications, for example in the drug development process [11,[17], [18], [19]], preclinical studies [20,21], various clinical applications [22], [23], [24], and elemental analysis [16]. Clinical applications such as disease diagnostics [22,23], TDM [25,26], whole exome sequencing [27], forensic toxicology study [28], as well as proteomics, genomics and metabolomics analysis [29], [30], [31], [32] have employed microsampling approaches to collect biological fluid samples from patients. Among these applications, the use of microsampling techniques in TDM has received great attention due to its feasibility and durability. TDM, as a branch of clinical pharmacology, involves the monitoring of drug dosage, adverse drug reaction and plasma concentration of patients after the administration of drugs. In order to obtain the desired therapeutic effect, drugs have to be administered in a suitable dosage and the steady state plasma concentration should always be within the therapeutic window. A too high or too low drug concentration in plasma may lead to detrimental toxic effects and therapeutic failure [33]. Patients who are taking prescribed drugs with narrow therapeutic windows or individualized medications are required to perform routine TDM to optimize drug dosage and ensure that the drug concentration in plasma is within the therapeutic range [33,34]. Common examples of drugs that are prescribed with the requirement of TDM are anti-epileptics [35], antipsychotics [36], anti-tuberculosis [37], antiviral [38], and immunosuppressant drugs [39].

This review article offers an overview of the recent advancements in microsampling techniques, particularly in the application of TDM, covering the period January 2015 – August 2020. As whole blood is the gold standard sampling matrix for TDM, the recent insights of microsampling techniques for whole blood, namely DBS and its advanced development, are the main focus of this review article. The application of TDM and the general aspects of the DBS approach, including sample collection, sample preparation, advanced developments, advantages and challenges will be extensively discussed. Other microsampling techniques that have evolved from DBS, including VAMS and DMS, will also be reviewed based on their application of TDM.