A power law function describes the time- and dose-dependency of Vγ9Vδ2 T cell activation by phosphoantigens

Enhanced Plasma Stability and Potency of Aryl/Acyloxy Prodrugs of a BTN3A1 Ligand

While ester-based phosphonate prodrugs excel at delivering payloads into cells, their instability in plasma is a hurdle for their advancement. Here, we synthesized new aryl/acyloxy prodrugs of a phosphonate BTN3A1 ligand. We evaluated their phosphoantigen potency by flow cytometry and ELISA and their plasma and cellular metabolism by LC-MS. These compounds displayed low nanomolar to high picomolar potency. Addition of a p-isopropyl group to the phenyl substituent and use of cyclohexyl or p-methoxybenzyl groups as the acyloxy substituent significantly increased human, but not mouse or rat, plasma stability without negatively impacting potency. Combinations of these prodrug moieties further improved stability, with the best combination achieving a half-life of over 12 h in human plasma, a marked improvement on prior compounds. In contrast, oxane analogs improved water solubility and cellular payload delivery but remained unstable in human plasma. The studies suggest that certain ester-based phosphonate prodrugs quickly deliver active payloads inside cells and show substantial stability in human plasma.

Synthesis and evaluation of (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate analogs as competitive partial agonists of butyrophilin 3A1

Phosphoantigens (pAgs) induce conformational changes after binding to the intracellular region of BTN3A1 which result in its clustering with BTN2A1, forming an activating ligand for the Vγ9Vδ2 T cell receptor. Here, we designed a small panel of bulky analogs of the prototypical pAg (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP) that contain an aromatic ring attached to the C-3 position in place of methyl group. These compounds bind with high affinity to BTN3A1 but fail to fully support its interaction with BTN2A1 and only partially trigger T cell activation relative to HMBPP. Furthermore, they can compete with HMBPP for cellular binding to BTN3A1 and reduce the cellular response to HMBPP, a classic partial agonist phenotype. Trifluoromethyl analog 6e was the weakest agonist but the strongest inhibitor of HMBPP ELISA response. Our study provides a rationale for the mode of action of pAg-induced γδ T cell activation and provides insights into other naturally occurring BTN proteins and their respective ligands.

Effective γδ T-cell clinical therapies: current limitations and future perspectives for cancer immunotherapy

γδ T cells are a unique subset of T lymphocytes, exhibiting features of both innate and adaptive immune cells and are involved with cancer immunosurveillance. They present an attractive alternative to conventional T cell-based immunotherapy due, in large part, to their lack of major histocompatibility (MHC) restriction and ability to secrete high levels of cytokines with well-known anti-tumour functions. To date, clinical trials using γδ T cell-based immunotherapy for a range of haematological and solid cancers have yielded limited success compared with in vitro studies. This inability to translate the efficacy of γδ T-cell therapies from preclinical to clinical trials is attributed to a combination of several factors, e.g. γδ T-cell agonists that are commonly used to stimulate populations of these cells have limited cellular uptake yet rely on intracellular mechanisms; administered γδ T cells display low levels of tumour-infiltration; and there is a gap in the understanding of γδ T-cell inhibitory receptors. This review explores the discrepancy between γδ T-cell clinical and preclinical performance and offers viable avenues to overcome these obstacles. Using more direct γδ T-cell agonists, encapsulating these agonists into lipid nanocarriers to improve their pharmacokinetic and pharmacodynamic profiles and the use of combination therapies to overcome checkpoint inhibition and T-cell exhaustion are ways to bridge the gap between preclinical and clinical success. Given the ability to overcome these limitations, the development of a more targeted γδ T-cell agonist-checkpoint blockade combination therapy has the potential for success in clinical trials which has to date remained elusive.

Diester Prodrugs of a Phosphonate Butyrophilin Ligand Display Improved Cell Potency, Plasma Stability, and Payload Internalization

Activation of Vγ9Vδ2 T cells with butyrophilin 3A1 (BTN3A1) agonists such as (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP) has the potential to boost the immune response. Because HMBPP is highly charged and metabolically unstable, prodrugs may be needed to overcome these liabilities, but the prodrugs themselves may be limited by slow payload release or low plasma stability. To identify effective prodrug forms of a phosphonate agonist of BTN3A1, we have prepared a set of diesters bearing one aryl and one acyloxymethyl group. The compounds were evaluated for their ability to stimulate Vγ9Vδ2 T cell proliferation, increase production of interferon γ, resist plasma metabolism, and internalize into leukemia cells. These bioassays have revealed that varied aryl and acyloxymethyl groups can decouple plasma and cellular metabolism and have a significant impact on bioactivity (>200-fold range) and stability (>10 fold range), including some with subnanomolar potency. Our findings increase the understanding of the structure-activity relationships of mixed aryl/acyloxymethyl phosphonate prodrugs.

Generation of effector Vγ9Vδ2 T cells and evaluation of their response to phosphoantigen-loaded cells

Vγ9Vδ2 T cells are non-canonical T cells that use their T cell receptor to detect phosphoantigens bound to the internal domain of the HMBPP receptor (butyrophilin 3/2A1 complex). This protocol describes the expansion and purification of human effector Vγ9Vδ2 T cells from human buffy coat and describes how to assess their activation by antigen-containing target cells. While specifically focused on cytokine production, this protocol can be readily adapted to evaluate other effector functions of activated Vγ9Vδ2 T cells. For complete details on the use and execution of this protocol, please refer to Hsiao et al. (2022) and Hsiao and Wiemer (2018).

Efficiency of bis-amidate phosphonate prodrugs

Bis-amidate derivatives have been viewed as attractive phosphonate prodrug forms because of their straightforward synthesis, lack of phosphorus stereochemistry, plasma stability and nontoxic amino acid metabolites. However, the efficiency of bis-amidate prodrug forms is unclear, as prior studies on this class of prodrugs have not evaluated their activation kinetics. Here, we synthetized a small panel of bis-amidate prodrugs of butyrophilin ligands as potential immunotherapy agents. These compounds were examined relative to other prodrug forms delivering the same payload for their stability in plasma and cell lysate, their ability to stimulate T cell proliferation in human PBMCs, and their activation kinetics in a leukemia co-culture model of T cell cytokine production. The bis-amidate prodrugs demonstrate high plasma stability and improved cellular phosphoantigen activity relative to the free phosphonic acid. However, the efficiency of bis-amidate activation is low relative to other prodrugs that contain at least one ester such as aryl-amidate, aryl-acyloxyalkyl ester, and bis-acyloxyalkyl ester forms. Therefore, bis-amidate prodrugs do not drive rapid cellular payload accumulation and they would be more useful for payloads in which slower, sustained-release kinetics are preferred.

Hydroxy- and Amino-Phosphonates and -Bisphosphonates: Synthetic Methods and Their Biological Applications

Phosphonates and bisphosphonates are stable analogs of phosphates and pyrophosphates that are characterized by one and two carbon–phosphorus bonds, respectively. Among the various phosphonates and bisphosphonates, hydroxy and amino substitutes are of interest as effective in medicinal and industrial chemistry. For example, hydroxy bisphosphonates have proven to be effective for the prevention of bone loss, especially in osteoporotic disease. On the other hand, different substitutions on the carbon atom connected to phosphorus have led to the synthesis of many different hydroxy- and amino-phosphonates and -bisphosphonates, each with its distinct physical, chemical, biological, therapeutic, and toxicological characteristics. Dialkyl or aryl esters of phosphonate and bisphosphonate compounds undergo the hydrolysis process readily and gave valuable materials with wide applications in pharmaceutical and agriculture. This review aims to demonstrate the ongoing preparation of various classes of hydroxy- and amino-phosphonates and -bisphosphonates. Furthermore, the current review summarizes and comprehensively describes articles on the biological applications of hydroxyl- and amino-phosphonates and -bisphosphonates from 2015 until today.

Prodrugs of pyrophosphates and bisphosphonates: disguising phosphorus oxyanions

Pyrophosphates have important functions in living systems and thus pyrophosphate-containing molecules and their more stable bisphosphonate analogues have the potential to be used as drugs for treating many diseases including cancer and viral infections. Both pyrophosphates and bisphosphonates are polyanionic at physiological pH and, whilst this is essential for their biological activity, it also limits their use as therapeutic agents. In particular, the high negative charge density of these compounds prohibits cell entry other than by endocytosis, prevents transcellular oral absorption and causes sequestration to bone. Therefore, prodrug strategies have been developed to temporarily disguise the charges of these compounds. This review examines the various systems that have been used to mask the phosphorus-containing moieties of pyrophosphates and bisphosphonates and also illustrates the utility of such prodrugs.

Synthesis and Metabolism of BTN3A1 Ligands: Studies on Diene Modifications to the Phosphoantigen Scaffold

Phosphoantigens (pAgs) are small organophosphorus compounds such as (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP) that trigger an immune response. These molecules bind to butyrophilin 3A1 (part of the HMBPP receptor) and activate Vγ9Vδ2 T cells. To explore the structure-activity relationships underlying this process, we evaluated a series of novel diene analogs of HMBPP. Here we report that prodrug forms of [(1E)-4-methylpenta-1,3-dien-1-yl] phosphonic acid that lack the allylic alcohol of HMBPP but instead contained a diene scaffold exhibit mid-nanomolar potency for the activation of Vγ9Vδ2 T cells. The compounds also trigger the production of T-cell interferon γ upon exposure to loaded K562 cells. Although both the allylic alcohol and the diene scaffold boost pAg activity, the combination of the two decreases the activity and results in glutathione conjugation. Together, these data show that the diene scaffold results in intermediate pAgs that may have implications for the mechanisms regulating the HMBPP receptor.

ZINC40099027 activates human focal adhesion kinase by accelerating the enzymatic activity of the FAK kinase domain

Focal adhesion kinase (FAK) regulates gastrointestinal epithelial restitution and healing. ZINC40099027 (Zn27) activates cellular FAK and promotes intestinal epithelial wound closure in vitro and in mice. However, whether Zn27 activates FAK directly or indirectly remains unknown. We evaluated Zn27 potential modulation of the key phosphatases, PTP-PEST, PTP1B, and SHP2, that inactivate FAK, and performed in vitro kinase assays with purified FAK to assess direct Zn27-FAK interaction. In human Caco-2 cells, Zn27-stimulated FAK-Tyr-397 phosphorylation despite PTP-PEST inhibition and did not affect PTP1B-FAK interaction or SHP2 activity. Conversely, in vitro kinase assays demonstrated that Zn27 directly activates both full-length 125 kDa FAK and its 35 kDa kinase domain. The ATP-competitive FAK inhibitor PF573228 reduced basal and ZN27-stimulated FAK phosphorylation in Caco-2 cells, but Zn27 increased FAK phosphorylation even in cells treated with PF573228. Increasing PF573228 concentrations completely prevented activation of 35 kDa FAK in vitro by a normally effective Zn27 concentration. Conversely, increasing Zn27 concentrations dose-dependently activated kinase activity and overcame PF573228 inhibition of FAK, suggesting the direct interactions of Zn27 with FAK may be competitive. Zn27 increased the maximal activity (Vmax ) of FAK. The apparent Km of the substrate also increased under laboratory conditions less relevant to intracellular ATP concentrations. These results suggest that Zn27 is highly potent and enhances FAK activity via allosteric interaction with the FAK kinase domain to increase the Vmax of FAK for ATP. Understanding Zn27 enhancement of FAK activity will be important to redesign and develop a clinical drug that can promote mucosal wound healing.

Metabolic Efficacy of Phosphate Prodrugs and the Remdesivir Paradigm

Drugs that contain phosphates (and phosphonates or phosphinates) have intrinsic absorption issues and are therefore often delivered in prodrug forms to promote their uptake. Effective prodrug forms distribute their payload to the site of the intended target and release it efficiently with minimal byproduct toxicity. The ability to balance unwanted payload release during transit with desired release at the site of action is critical to prodrug efficacy. Despite decades of research on prodrug forms, choosing the ideal prodrug form remains a challenge which is often solved empirically. The recent emergency use authorization of the antiviral remdesivir for COVID-19 exemplifies a new approach for delivery of phosphate prodrugs by parenteral dosing, which minimizes payload release during transit and maximizes tissue payload distribution. This review focuses on the role of metabolic activation in efficacy during oral and parenteral dosing of phosphate, phosphonate, and phosphinate prodrugs. Through examining prior structure–activity studies on prodrug forms and the choices that led to development of remdesivir and other clinical drugs and drug candidates, a better understanding of their ability to distribute to the planned site of action, such as the liver, plasma, PBMCs, or peripheral tissues, can be gained. The structure–activity relationships described here will facilitate the rational design of future prodrugs.

Structure-Activity Relationships of Butyrophilin 3 Ligands

Phosphoantigens (pAgs) are small phosphorus-containing molecules that stimulate Vγ9Vδ2 T cells with sub-nanomolar cellular potency. Recent work has revealed that these compounds work through binding to the transmembrane immunoglobulin butyrophilin 3A1 (BTN3A1) within its intracellular B30.2 domain. Engagement of BTN3A1 is critical to the formation of an immune synapse between cells that contain pAgs and the Vγ9Vδ2 T cells. This minireview summarizes the structure-activity relationships of pAgs and their implications to the mechanisms of butyrophilin 3 activation leading to Vγ9Vδ2 T cell response.

A luciferase lysis assay reveals in vivo malignant cell sensitization by phosphoantigen prodrugs

Human Vγ9Vδ2 T cells respond to small phosphorus-containing compounds, often called phosphoantigens, which are now known to be intracellular ligands of the immune receptor butyrophilin 3A1 (BTN3A1). In order to compare the efficiency of butyrophilin ligands, we developed a luciferase-based lysis assay that measures the direct cytolysis by Vγ9Vδ2 T cells of luciferase-expressing K562 leukemia cells sensitized by phosphoantigen prodrugs. Our results show that the luciferase-based lysis assay allows in vitro and in vivo assessment of phosphoantigen activity in a way that does not require the extensive processing of flow cytometry or ELISA based approaches. In cellular assays, the structure activity relationships of phosphoantigen prodrugs correlate with ELISA-based activation assays, though phosphoantigen induced target cell lysis occurs at lower concentrations relative to T cell interferon γ production measured by ELISA. In mice dosed with phosphoantigens, a racemic aryl phosphonamidate prodrug, methyl 2-[[[(E)-5-hydroxy-4-methyl-pent-3-enyl]-(1-naphthyloxy)phosphoryl]amino]acetate (1-Nap/GlyOMe C-HMBP, 5), sensitized subcutaneous K562 tumors within minutes, and this effect was maintained at least four hours after treatment. In vivo activity of compound 5 was stronger than that of an equivalent dose of zoledronate. This luciferase lysis assay can be used for evaluation of phosphoantigens due to its time efficiency, high sensitivity, and in vivo compatibility and demonstrates rapid in vitro and in vivo sensitization of tumor cells by phosphoantigen prodrugs.

Synthesis and Bioactivity of the Alanyl Phosphonamidate Stereoisomers Derived from a Butyrophilin Ligand

Aryloxy phosphonamidate derivatives of a butyrophilin 3A1 ligand are stimulants of Vγ9 Vδ2 T cells. However, when bonded to an aryl ester and an amine, the phosphorus is stereogenic, and past compounds were studied as racemates. To determine the impact of stereochemistry on the activity, we now have prepared phosphonate derivatives of l- and d-alanine ethyl ester, separated the diastereomers, and evaluated their biological activity as single stereoisomers. The results demonstrate that phosphonamidates substituted with l-alanine stimulate Vγ9 Vδ2 T cells at lower concentrations than the racemic glycine counterpart, while those derived from d-alanine require higher concentrations. All four diastereomers are more active than charged phosphoantigens such as HMBPP. Surprisingly, only a 2-fold difference was observed between the l-alanine phosphorus isomers, with the R _P isomer more potent. This suggests that the small phosphoantigen scaffold reduces but does not eliminate dependence upon phosphorus stereochemistry for cellular activity.

Stability and Efficiency of Mixed Aryl Phosphonate Prodrugs

A set of phosphonate prodrugs of a butyrophilin ligand was synthesized and evaluated for plasma stability and cellular activity. The mixed aryl acyloxy esters were prepared either via a standard sequence through the phosphonic acid chloride, or through the more recently reported, and more facile, triflate activation. In the best of cases, this class of prodrugs shows cellular potency similar to that of bis-acyloxyalkyl phosphonate prodrugs and plasma stability similar to that of aryl phosphonamidates. For example, {[((3E)-5-hydroxy-4-methylpent-3-en-1-yl) (naphthalen-2-yloxy)phosphoryl]oxy}methyl 2,2-dimethylpropanoate can activate BTN3A1 in K562 cells after just 15 minutes of exposure (at an EC50 value of 31 nm) and is only partially metabolized (60 % remaining) after 20 hours in human plasma. Other related novel analogues showed similar potency/stability profiles. Therefore, mixed aryl acyloxyalkyl phosphonate prodrugs are an exciting new strategy for the delivery of phosphonate-containing drugs.

Probing the Ligand-Binding Pocket of BTN3A1

Small-molecule phosphoantigens such as (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate stimulate human Vγ9Vδ2 T cells after binding to the intracellular B30.2 domain of the immune receptor butyrophilin 3 isoform A1 (BTN3A1). To understand the ligand-target interaction in greater detail, we performed molecular docking. Based on the docking results, we synthesized the novel ligand (E)-(7-hydroxy-6-methylhept-5-en-1-yl)phosphonate and mutated proposed binding site residues. We evaluated the impact on butyrophilin binding of existing and novel ligands using a newly developed high-throughput fluorescence polarization assay. We also evaluated the ability of the compounds to stimulate proliferation and interferon-γ production of Vγ9Vδ2 T cells. Mutation of H381 fully blocked ligand binding, whereas mutations to charged surface residues impacted diphosphate interactions. Monophosphonate analogs bind similarly to BTN3A1, although they differ in their antigenicity, demonstrating that binding and efficacy are not linearly correlated. These results further define the structure-activity relationships underlying BTN3A1 ligand binding and antigenicity and support further structure-guided drug design.