MALDI Mass Spectrometry Imaging Reveals Heterogeneous Distribution of Tenofovir and Tenofovir Diphosphate in Colorectal Tissue of Subjects Receiving a Tenofovir-Containing Enema

Biotransformation research advances - 2023 year in review

This annual review marks the eighth in the series starting with Baillie et al. (2016) Our objective is to explore and share articles which we deem influential and significant in the field of biotransformation. Its format is to highlight important aspects captured in synopsis followed by a commentary with relevant figure and references.

Spatial Heterogeneity of Brain Lipids in SIV-Infected Macaques Treated with Antiretroviral Therapy

Human immunodeficiency virus (HIV) infection continues to promote neurocognitive impairment, mood disorders, and brain atrophy, even in the modern era of viral suppression. Brain lipids are vulnerable to HIV-associated energetic strain and may contribute to HIV-associated neurologic dysfunction due to alterations in lipid breakdown and structural lipid composition. HIV neuropathology is region dependent, yet there has not been comprehensive characterization of the spatial heterogeneity of brain lipids during infection that possibly impacts neurologic function. To address this gap, we evaluated the spatial lipid distribution using matrix laser desorption/ionization imaging mass spectrometry (MALDI-IMS) across four brain regions (parietal cortex, midbrain, thalamus, and temporal cortex), as well as the kidney for a peripheral tissue control, in a simian immunodeficiency virus (SIV)-infected rhesus macaque treated with a course of antiretroviral therapies (ARTs). We assessed lipids indicative of fat breakdown [acylcarnitines (CARs)] and critical structural lipids [phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs)] across fatty acid chain lengths and degrees of unsaturation. CARs with very long-chain, polyunsaturated fatty acids (PUFAs) were more abundant across all brain regions than shorter chain, saturated, or monounsaturated species. We observed distinct brain lipid distribution patterns for the CARs and PCs. However, no clear expression patterns emerged for PEs. Surprisingly, the kidney was nearly devoid of ions corresponding to PUFAs common in brain. PEs and PCs with PUFAs had little intensity and less density than other species, and only one CAR species was observed in kidney at high intensity. Overall, our study demonstrates the stark variation in structural phospholipids and lipid-energetic intermediates present in the virally suppressed SIV-macaque brain. These findings may be useful for identifying regional vulnerabilities to damage due to brain lipid changes in people with HIV.

Recent Advances in Mass Spectrometry-Based Spatially Resolved Molecular Imaging of Drug Disposition and Metabolomics

Mass spectrometric imaging is a nontargeted, tag-free, high-throughput, and highly responsive analytical approach. The highly accurate molecular visualization detection technology enables qualitative and quantitative analyses of biologic tissues or cells scanned by mass spectrometry in situ, extracting known and unknown multiple compounds, and simultaneously assessing relative contents of targeting molecules by monitoring their molecular ions and pinpointing the spatial locations of those molecules distributed. Five mass spectrometric imaging techniques and their characteristics are introduced in the review, including matrix-assisted laser desorption ionization mass spectrometry, secondary ion mass spectrometry, desorption electrospray ionization mass spectrometry, laser ablation electrospray ionization mass spectrometry, and laser ablation inductively coupled plasma mass spectrometry. The mass spectrometry-based techniques provide the possibility for spatial metabolomics with the capability of high throughput and precision detection. The approaches have been widely employed to spatially image not only metabolome of endogenous amino acids, peptides, proteins, neurotransmitters, and lipids but also the disposition of exogenous chemicals, such as pharmaceutical agents, environmental pollutants, toxicants, natural products, and heavy metals. The techniques also provide us with spatial distribution imaging of analytes in single cells, tissue microregions, organs, and whole animals. SIGNIFICANCE STATEMENT: The review article includes an overview of five commonly used mass spectrometers for spatial imaging and describes the advantages and disadvantages of each. Examples of the technology applications cover drug disposition, diseases, and omics. Technical aspects of relative and absolute quantification by mass spectrometric imaging and challenges for future new applications are discussed as well. The reviewed knowledge may benefit the development of new drugs and provide a better understanding of biochemical processes related to physiology and diseases.

Nucleoside Triphosphate Diphosphohydrolase 1 Exhibits Enzymatic Activity toward Tenofovir Diphosphate

Tenofovir (TFV; prescribed as TFV disoproxil fumarate and TFV alafenamide prodrugs) is currently used for HIV prevention and treatment. TFV must be phosphorylated twice into TFV-diphosphate (TFV-DP) to become pharmacologically active. Previously, we reported heterogeneity in TFV-DP distribution in colorectal tissue (a putative site of HIV infection) sections collected from research participants receiving a TFV-containing enema. This observed heterogeneity is likely multifactorial. Of note, TFV-DP is structurally similar to ATP. It is known that nucleotidases such as nucleoside triphosphate diphosphohydrolases (NTPDases) dephosphorylate ATP. Thus, it was hypothesized that NTPDase-mediated dephosphorylation plays a role in regulating TFV-DP levels in colorectal tissue. To test this hypothesis, recombinant NTPDase proteins (NTPDase 1, 3, 4, 5, 6, and 8) were incubated, individually, with TFV-DP to determine their abilities to dephosphorylate TFV-DP in vitro. Following incubations, TFV-DP dephosphorylation was determined using both malachite green phosphate assays and ultrahigh-performance liquid chromatography tandem mass spectrometry. From these, NTPDase 1 exhibited the highest activity toward TFV-DP. Further, enzyme kinetic analysis revealed Michaelis-Menten kinetics for NTPDase 1-mediated TFV-DP dephosphorylation. Next, immunoblot analyses were conducted to confirm the expression of NTPDase 1 protein in human colorectal tissue. Liquid chromatography coupled to mass spectrometry proteomics analysis was used to measure the relative abundance of NTPDases in human colorectal tissue among healthy adult individuals (n = 4). These analyses confirmed the high abundance of NTPDase 1 in human colorectal tissue. Taken together, results suggest that NTPDase 1 may contribute to the regulation of TFV-DP levels. The above data provide important insights into the dephosphorylation of TFV-DP. SIGNIFICANCE STATEMENT: Nucleoside triphosphate diphosphohydrolases (NTPDases) that are involved in enzymatic ATP dephosphorylation may contribute to tenofovir-diphosphate (TFV-DP) dephosphorylation, leading to its inactivation. In this study, the NTPDases responsible for TFV-DP dephosphorylation in vitro and their expression in human colorectal tissue were investigated. Through this work, it was demonstrated that NTPDase 1 has the highest activity toward TFV-DP dephosphorylation, and it was abundant in human colorectal tissue. Importantly, these studies will increase our understanding of TFV-DP disposition.

Achieving a Deeper Understanding of Drug Metabolism and Responses Using Single-Cell Technologies

Recent advancements in single-cell technologies have enabled detection of RNA, proteins, metabolites, and xenobiotics in individual cells, and the application of these technologies has the potential to transform pharmacological research. Single-cell data has already resulted in the development of human and model species cell atlases, identifying different cell types within a tissue, further facilitating the characterization of tumor heterogeneity, and providing insight into treatment resistance. Research discussed in this review demonstrates that distinct cell populations express drug metabolizing enzymes to different extents, indicating there may be variability in drug metabolism not only between organs, but within tissue types. Additionally, we put forth the concept that single-cell analyses can be used to expose underlying variability in cellular response to drugs, providing a unique examination of drug efficacy, toxicity, and metabolism. We will outline several of these techniques: single-cell RNA-sequencing and mass cytometry to characterize and distinguish different cell types, single-cell proteomics to quantify drug metabolizing enzymes and characterize cellular responses to drug, capillary electrophoresis-ultrasensitive laser-induced fluorescence detection and single-probe single-cell mass spectrometry for detection of drugs, and others. Emerging single-cell technologies such as these can comprehensively characterize heterogeneity in both cell-type-specific drug metabolism and response to treatment, enhancing progress toward personalized and precision medicine. SIGNIFICANCE STATEMENT: Recent technological advances have enabled the analysis of gene expression and protein levels in single cells. These types of analyses are important to investigating mechanisms that cannot be elucidated on a bulk level, primarily due to the variability of cell populations within biological systems. Here, we summarize cell-type-specific drug metabolism and how pharmacologists can utilize single-cell approaches to obtain a comprehensive understanding of drug metabolism and cellular heterogeneity in response to drugs.

Atp7b-dependent choroid plexus dysfunction causes transient copper deficit and metabolic changes in the developing mouse brain

Copper (Cu) has a multifaceted role in brain development, function, and metabolism. Two homologous Cu transporters, Atp7a (Menkes disease protein) and Atp7b (Wilson disease protein), maintain Cu homeostasis in the tissue. Atp7a mediates Cu entry into the brain and activates Cu-dependent enzymes, whereas the role of Atp7b is less clear. We show that during postnatal development Atp7b is necessary for normal morphology and function of choroid plexus (ChPl). Inactivation of Atp7b causes reorganization of ChPl' cytoskeleton and cell-cell contacts, loss of Slc31a1 from the apical membrane, and a decrease in the length and number of microvilli and cilia. In ChPl lacking Atp7b, Atp7a is upregulated but remains intracellular, which limits Cu transport into the brain and results in significant Cu deficit, which is reversed only in older animals. Cu deficiency is associated with down-regulation of Atp7a in locus coeruleus and catecholamine imbalance, despite normal expression of dopamine-β-hydroxylase. In addition, there are notable changes in the brain lipidome, which can be attributed to inhibition of diacylglyceride-to-phosphatidylethanolamine conversion. These results identify the new role for Atp7b in developing brain and identify metabolic changes that could be exacerbated by Cu chelation therapy.

Fundamental aspects of long-acting tenofovir alafenamide delivery from subdermal implants for HIV prophylaxis

Global efforts aimed at preventing human immunodeficiency virus type one (HIV-1) infection in vulnerable populations appear to be stalling, limiting our ability to control the epidemic. Long-acting, controlled drug administration from subdermal implants holds significant potential by reducing the compliance burden associated with frequent dosing. We, and others, are exploring the development of complementary subdermal implant technologies delivering the potent prodrug, tenofovir alafenamide (TAF). The current report addresses knowledge gaps in the preclinical pharmacology of long-acting, subdermal TAF delivery using several mouse models. Systemic drug disposition during TAF implant dosing was explained by a multi-compartment pharmacokinetic (PK) model. Imaging mass spectrometry was employed to characterize the spatial distribution of TAF and its principal five metabolites in local tissues surrounding the implant. Humanized mouse studies determined the effective TAF dose for preventing vaginal and rectal HIV-1 acquisition. Our results represent an important step in the development of a safe and effective TAF implant for HIV-1 prevention.

Applications of mass spectrometry imaging in virus research

Mass spectrometry imaging (MSI) is a label-free molecular imaging technique allowing an untargeted detection of a broad range of biomolecules and xenobiotics. MSI enables imaging of the spatial distribution of proteins, peptides, lipids and metabolites from a wide range of samples. To date, this technique is commonly applied to tissue sections in cancer diagnostics and biomarker development, but also molecular histology in general. Advances in the methodology and bioinformatics improved the resolution of MS images below the single cell level and increased the flexibility of the workflow. However, MSI-based research in virology is just starting to gain momentum and its full potential has not been exploited yet. In this review, we discuss the main applications of MSI in virology. We review important aspects of matrix-assisted laser desorption/ionization (MALDI) MSI, the most widely used MSI technique in virology. In addition, we summarize relevant literature on MSI studies that aim to unravel virus-host interactions and virus pathogenesis, to elucidate antiviral drug kinetics and to improve current viral disease diagnostics. Collectively, these studies strongly improve our general understanding of virus-induced changes in the proteome, metabolome and metabolite distribution in host tissues of humans, animals and plants upon infection. Furthermore, latest MSI research provided important insights into the drug distribution and distribution kinetics, especially in antiretroviral research. Finally, MSI-based investigations of oncogenic viruses greatly increased our knowledge on tumor mass signatures and facilitated the identification of cancer biomarkers.

Integrating High-Resolution MALDI Imaging into the Development Pipeline of Anti-Tuberculosis Drugs

Successful treatment of tuberculosis (TB) requires antibiotics to reach their intended point of action, i.e., necrotizing granulomas in the lung. MALDI mass spectrometry imaging (MSI) is able to visualize the distribution of antibiotics in tissue, but resolving the small histological structures in mice, which are most commonly used in preclinical trials, requires high spatial resolution. We developed a MALDI MSI method to image antibiotics in the mouse lung with high mass resolution (240k @ m/z 200 fwhm) and high spatial resolution (10 μm pixel size). A crucial step was to develop a cryosectioning protocol that retains the distribution of water-soluble drugs in small and fragile murine lung lobes without inflation or embedding. Choice and application of matrices were optimized to detect human-equivalent drug concentrations in tissue, and measurement parameters were optimized to detect multiple drugs in a single tissue section. We succeeded in visualizing the distribution of all current first-line anti-TB drugs (pyrazinamide, rifampicin, ethambutol, isoniazid) and the second-line drugs moxifloxacin and clofazimine. Four of these compounds were imaged for the first time in the mouse lung. Accurate mass identification was confirmed by on-tissue MS/MS. Evaluation of fragmentation pathways revealed the structure of the double-protonated molecular ion of pyrazinamide. Clofazimine was imaged for the first time with 10 μm pixel size revealing clofazimine accumulation in lipid deposits around airways. In summary, we developed a platform to resolve the detailed histology in the murine lung and to reliably detect a range of anti-TB drugs at human-equivalent doses. Our workflow is currently being employed in preclinical mouse studies to evaluate the efficacy of novel anti-TB drugs.

Spatial Distribution Profiles of Emtricitabine, Tenofovir, Efavirenz, and Rilpivirine in Murine Tissues Following In Vivo Dosing Correlate with Their Safety Profiles in Humans

Emtricitabine (FTC), tenofovir (TFV), efavirenz (EFV), and rilpivirine (RPV) are currently used as components of HIV combination therapy. Although these drugs are widely used in antiretroviral therapy, several organ toxicities related to TFV and EFV have been observed clinically. TFV is associated with nephrotoxicity, whereas EFV-related hepatotoxicity and neurotoxicity have been reported. While the precise molecular mechanisms related to the above-mentioned clinically observed toxicities have yet to be elucidated, understanding the local tissue distribution profiles of these drugs could yield insights into their safety profiles. To date, the distributions of these drugs in tissue following in vivo exposure are poorly understood. Therefore, in this study, we employed a matrix-assisted laser desorption/ionization mass spectrometry imaging method to generate spatial distribution profiles of FTC, TFV, EFV, and RPV in mouse tissues following in vivo dosing of following drug regimens: TFV–FTC–EFV and TFV–FTC–RPV. For this study, liver, brain, kidney, spleen, and heart tissues were obtained from mice (n = 3) following separate oral administration of the above-mentioned drug regimens. Interestingly, EFV was detected in liver, brain, and heart following TFV–FTC–EFV treatment. Additionally, hydroxylated EFV, which encompasses the cytochrome P450-dependent monooxygenated metabolites of EFV, was detected in liver, brain, spleen, and heart tissue sections. Notably, the tissue distribution profiles of RPV and hydroxylated RPV following in vivo dosing of TFV–FTC–RPV were different from EFV/hydroxylated EFV despite RPV belonging to the same drug class as EFV. In conclusion, the observed spatial distribution profiles of the study drugs are in agreement with their safety profiles in humans.

Spatial distribution of elvitegravir and tenofovir in rat brain tissue: Application of matrix-assisted laser desorption/ionization mass spectrometry imaging and liquid chromatography/tandem mass spectrometry

RATIONALE

The complexity of central nervous system (CNS) drug delivery is the main obstacle with the blood-brain barrier (BBB) known to restrict access of most pharmaceutical drugs into the brain. Mass spectrometry imaging (MSI) offers possibilities for studying drug deposition into the CNS.

METHODS

The deposition and spatial distribution of the two antiretroviral drugs elvitegravir and tenofovir in the brain were investigated in healthy female Sprague-Dawley rats following a single intraperitoneal administration (50 mg/kg). This was achieved by the utilization of quantitative liquid chromatography/tandem mass spectrometry (LC/MS/MS) and matrix-assisted laser desorption/ionization (MALDI) MSI.

RESULTS

LC/MS/MS showed that elvitegravir has better BBB penetration, reaching maximum concentration in the brain (C_max brain) of 976.5 ng/g. In contrast, tenofovir displayed relatively lower BBB penetration, reaching C_max brain of 54.5 ng/g. MALDI-MSI showed the heterogeneous distribution of both drugs in various brain regions including the cerebral cortex.

CONCLUSIONS

LC/MS/MS and MALDI-MSI provided valuable information about the relative concentration and the spatial distribution of the two common antiretroviral drugs. This study has also shown the capability of MALDI-MSI for direct visualization of pharmaceutical drugs in situ.

Drug Concentration Asymmetry in Tissues and Plasma for Small Molecule-Related Therapeutic Modalities

The well accepted "free drug hypothesis" for small-molecule drugs assumes that only the free (unbound) drug concentration at the therapeutic target can elicit a pharmacologic effect. Unbound (free) drug concentrations in plasma are readily measurable and are often used as surrogates for the drug concentrations at the site of pharmacologic action in pharmacokinetic-pharmacodynamic analysis and clinical dose projection in drug discovery. Furthermore, for permeable compounds at pharmacokinetic steady state, the free drug concentration in tissue is likely a close approximation of that in plasma; however, several factors can create and maintain disequilibrium between the free drug concentration in plasma and tissue, leading to free drug concentration asymmetry. These factors include drug uptake and extrusion mechanisms involving the uptake and efflux drug transporters, intracellular biotransformation of prodrugs, membrane receptor-mediated uptake of antibody-drug conjugates, pH gradients, unique distribution properties (covalent binders, nanoparticles), and local drug delivery (e.g., inhalation). The impact of these factors on the free drug concentrations in tissues can be represented by K _p,uu, the ratio of free drug concentration between tissue and plasma at steady state. This review focuses on situations in which free drug concentrations in tissues may differ from those in plasma (e.g., K _p,uu > or <1) and discusses the limitations of the surrogate approach of using plasma-free drug concentration to predict free drug concentrations in tissue. This is an important consideration for novel therapeutic modalities since systemic exposure as a driver of pharmacologic effects may provide limited value in guiding compound optimization, selection, and advancement. Ultimately, a deeper understanding of the relationship between free drug concentrations in plasma and tissues is needed.

Genetic variation of kinases and activation of nucleotide analog reverse transcriptase inhibitor tenofovir

As antiretroviral therapy has become more accessible across the world and coformulations have improved patient compliance; the morbidity and mortality of HIV/AIDS has decreased. However, there is still a substantial gap in knowledge regarding the impact of genetic variation on the metabolism of and response to some of the most commonly prescribed antiretrovirals, including the nucleotide reverse transcriptase inhibitor tenofovir. While it has been scientifically established that tenofovir must be activated to be efficacious against HIV, the enzymes responsible for this activation have not been well characterized. The purpose of this review is to summarize and clarify the scientific knowledge regarding the enzymes that phosphorylate and activate this clinically important drug.