Hydrophilic Small Molecules That Harness Transthyretin To Enhance the Safety and Efficacy of Targeted Chemotherapeutic Agents

Emerging non-antibody‒drug conjugates (non-ADCs) therapeutics of toxins for cancer treatment

The non-selective cytotoxicity of toxins limits the clinical relevance of the toxins. In recent years, toxins have been widely used as warheads for antibody‒drug conjugates (ADCs) due to their efficient killing activity against various cancer cells. Although ADCs confer certain targeting properties to the toxins, low drug loading capacity, possible immunogenicity, and other drawbacks also limit the potential application of ADCs. Recently, non-ADC delivery strategies for toxins have been extensively investigated. To further understand the application of toxins in anti-tumor, this paper provided an overview of prodrugs, nanodrug delivery systems, and biomimetic drug delivery systems. In addition, toxins and their combination strategies with other therapies were discussed. Finally, the prospect and challenge of toxins in cancer treatment were also summarized.

Design and Preclinical Evaluation of a Novel Prostate-Specific Membrane Antigen Radioligand Modified with a Transthyretin Binder

Transthyretin binders have previously been used to improve the pharmacokinetic properties of small-molecule drug conjugates and could, thus, be utilized for radiopharmaceuticals as an alternative to the widely explored "albumin binder concept". In this study, a novel PSMA ligand modified with a transthyretin-binding entity (TB-01) was synthesized and labeled with lutetium-177 to obtain [177Lu]Lu-PSMA-TB-01. A high and specific uptake of [177Lu]Lu-PSMA-TB-01 was found in PSMA-positive PC-3 PIP cells (69 ± 3% after 4 h incubation), while uptake in PSMA-negative PC-3 flu cells was negligible (<1%). In vitro binding studies showed a 174-fold stronger affinity of [177Lu]Lu-PSMA-TB-01 to transthyretin than to human serum albumin. Biodistribution studies in PC-3 PIP/flu tumor-bearing mice confirmed the enhanced blood retention of [177Lu]Lu-PSMA-TB-01 (16 ± 1% IA/g at 1 h p.i.), which translated to a high tumor uptake (69 ± 13% IA/g at 4 h p.i.) with only slow wash-out over time (31 ± 8% IA/g at 96 h p.i.), while accumulation in the PC-3 flu tumor and non-targeted normal tissue was reasonably low. Further optimization of the radioligand design would be necessary to fine-tune the biodistribution and enable its use for therapeutic purposes. This study was the first of this kind and could motivate the use of the "transthyretin binder concept" for the development of future radiopharmaceuticals.

Targeted Therapy for Prostate Cancer by Prostate-Specific Membrane Antigen-Targeted Small-Molecule Drug Conjugates

In the aging global population, prostate cancer is a worldwide health problem because the incidence rate of this disease increases at advanced ages. Although early-stage prostate cancer can be treated by total prostatectomy, the surgery causes side effects, such as incontinence and dysuria, that lower QOL. Once the disease progresses to metastatic castration-resistant prostate cancer (mCRPC), there are no effective chemotherapeutic agents without systematic side effects. Therefore, targeted therapies for mCPRC are urgently needed. Traditional antibody-drug conjugate treatments for prostate cancer have been tested in clinical trials and several side effects have been observed. Meanwhile, small-molecule drug conjugates (SMDCs) have certain advantages over antibody drug conjugates in terms of non-immunogenicity, reproducibility, and permeability. In this review, prostate-specific membrane antigen-targeted SMDCs for treating prostate cancer are summarized.

Advances in non-radioactive PSMA-targeted small molecule-drug conjugates in the treatment of prostate cancer

Most patients with advanced prostate cancer (PCa) will develop metastatic castration-resistant prostate cancer (mCRPC) after androgen deprivation therapy, at this time the tumor enters the end stage, and the clinical treatment is very complicated, which requires rationalization of drugs to prolong the life of patients while improving their quality of life. Prostate-specific membrane antigen (PSMA) is a promising biological target for drug delivery in mCRPC due to its high level of specific expression in PCa cell membranes and low expression in normal tissues. Non-radioactive PSMA-targeted small molecule-drug conjugates (SMDCs) are gradually becoming a heat of discovery due to their good affinity and specificity; simple synthesis steps and transport management methods. Non-radioactive PSMA-targeted SMDCs under investigation can be divided into two categories: SMDCs and dual-ligand coupled drugs, among which SMDCs are the most widespread form of this type of conjugate. SMDCs have three key components: cytotoxic load, linker, and small molecule targeting ligands. SMDCs are internalized into the cell after binding to PSMA on the cell membrane and stored in endosomes and lysosomes, where they are usually enzymatically cleaved to allow precise release of cytotoxic molecules and uniform diffusion into the tumor tissue. More than a dozen non-radioactive PSMA-targeted SMDCs have been developed, many of which have shown favorable properties in both in vitro and in vivo evaluations, demonstrating more favorable results than unmodified cytotoxic drugs. Therefore, non-radioactive PSMA-targeted SMDCs have great therapeutic potential for mCRPC as a form of targeted therapy.

Combining Solid-State NMR with Structural and Biophysical Techniques to Design Challenging Protein-Drug Conjugates

Several protein-drug conjugates are currently being used in cancer therapy. These conjugates rely on cytotoxic organic compounds that are covalently attached to the carrier proteins or that interact with them via non-covalent interactions. Human transthyretin (TTR), a physiological protein, has already been identified as a possible carrier protein for the delivery of cytotoxic drugs. Here we show the structure-guided development of a new stable cytotoxic molecule based on a known strong binder of TTR and a well-established anticancer drug. This example is used to demonstrate the importance of the integration of multiple biophysical and structural techniques, encompassing microscale thermophoresis, X-ray crystallography and NMR. In particular, we show that solid-state NMR has the ability to reveal effects caused by ligand binding which are more easily relatable to structural and dynamical alterations that impact the stability of macromolecular complexes.

Enhancing the Safety and Efficacy of PSMA-Based Small-Molecule Drug Conjugates by Linker Stabilization and Conjugation to Transthyretin Binding Ligand

This work describes the enhancement of a novel antitumor therapeutic platform that combines advantages from small-molecule drug conjugates (SMDCs) and antibody drug conjugates (ADCs). Valine-citrulline (VCit) dipeptide linkers are commonly used cathepsin B cleavable linkers for ADCs. However, the instability of these linkers in mouse serum makes translating efficacy data from mouse to human more challenging. Replacing the VCit linker with glutamic acid-valine-citrulline (EVCit) has been reported to enhance the stability of ADCs in mouse serum. However, the effect of EVCit linker on the stability of SMDCs has not been reported. Here, we report that incorporating the EVCit linker in prostate-specific membrane antigen-targeting SMDCs, equipped with the transthyretin ligand AG10, resulted in conjugates with lower toxicity, an extended half-life, and superior therapeutic efficacy to docetaxel in a xenograft mouse model of prostate cancer. This should make SMDCs' preclinical toxicity and efficacy data from mice more reliable for predicting human results.

Extending the in vivo persistence of synthetic glycoconjugates using a serum-protein binder

Synthetic glycoconjugates are used in the development of vaccines and the design of inhibitors for glycan-protein interactions. The in vivo persistence of synthetic glycoconjugates is an important factor in their efficacy, especially when prolonged interactions with specific cell types may be required. In this study, we applied a strategy for non-covalent association of an active compound with serum proteins for extension of glycoconjugate half-life in serum. The small molecule, AG10, has previously been used to extend the half-life of small molecules through its high affinity for transthyretin (TTR), a serum protein. Using a tetravalent polyethylene glycol (PEG)-based scaffold we developed a synthetic strategy for glycoconjugates that allowed for controlled addition of multiple tags, such as a TTR affinity tag or fluorophore. We designed a version of AG10 modified at the pyrazole core, named GD10, amenable to our conjugation strategy and introduced to glycoconjugates using a tri-functional linker. This approach allowed for attachment of GD10 and fluorophore tags, as well as carbohydrate antigens. We then tested the influence of the GD10 tag on glycoconjugate half-life in vivo using a mouse model. Our results suggest that the combination of the GD10 tag and the PEG scaffold extended the half-life of glycoconjugates by as much as 10-fold when compared to proteins of similar molecular weight. The GD10 tag was able to extend the half-life of similar glycoconjugates by as much as 2-fold. We observed a role for the terminal saccharide residue of the carbohydrate antigen and confirmed that conjugates were able to penetrate multiple compartments in vivo including bone marrow, lymph nodes, and other organs. The introduction of the GD10 tag did not obstruct the ability of conjugates to interact with lectin receptors. We conclude that serum protein binders can be used to extend the persistence of glycoconjugates in vivo.

Peripherally restricted transthyretin-based delivery system for probes and therapeutics avoiding opioid-related side effects

Several investigations into the sites of action of opioid analgesics have utilized peripherally acting mu-opioid receptor antagonists (PAMORAs), which have been incorrectly assumed to possess limited permeability across the blood-brain barrier. Unfortunately, the poor pharmacokinetic properties of current PAMORAs have resulted in misunderstandings of the role of central nervous system and gastrointestinal tract in precipitating side effects such as opioid-induced constipation. Here, we develop a drug delivery approach for restricting the passage of small molecules across the blood-brain barrier. This allows us to develop naloxone- and oxycodone-based conjugates that display superior potency, peripheral selectivity, pharmacokinetics, and efficacy in rats compared to other clinically used PAMORAs. These probes allow us to demonstrate that the mu-opioid receptors in the central nervous system have a fundamental role in precipitating opioid-induced constipation. Therefore, our conjugates have immediate use as pharmacological probes and potential therapeutic agents for treating constipation and other opioid-related side effects.

Synthesis, Characterization, and Preclinical Evaluation of a Small-Molecule Prostate-Specific Membrane Antigen-Targeted Monomethyl Auristatin E Conjugate

Prostate cancer is the second most common type of cancer among men. Its main method of treatment is chemotherapy, which has a wide range of side effects. One of the solutions to this challenge is targeted delivery to prostate cancer cells. Here we synthesized a novel small-molecule PSMA-targeted conjugate based on the monomethyl auristatin E. Its structure and conformational properties were investigated by NMR spectroscopy. Cytotoxicity, intracellular reactive oxygen species induction, and stability under liver microsomes and P450-cytochrome species were investigated for this conjugate. The conjugate demonstrated 77-85% tumor growth inhibition levels on 22Rv1 (PSMA (+)) xenografts, compared with a 37% inhibition level on PC-3 (PSMA (-)) xenografts, in a single dose of 0.3 mg/kg and a sufficiently high therapeutic index of 21. Acute, chronic, and subchronic toxicities and pharmacokinetics have shown that the synthesized conjugate is a promising potential agent for the chemotherapy of prostate cancer.

PSMA-targeted small-molecule docetaxel conjugate: Synthesis and preclinical evaluation

Prostate cancer is one of the most commonly diagnosed men's cancers and remains one of the leading causes of cancer death. The development of approaches to the treatment of this oncological disease is an ongoing process. In this work, we have carried out the selection of ligands for the creation of conjugates based on the drug docetaxel and synthesized a series of three docetaxel conjugates. In vitro cytotoxicity of these molecules was evaluated using the MTT assay. Based on the assay results, we selected the conjugate which showed cytotoxic potential close to unmodified docetaxel. At the same time, the molar solubility of the resulting compound increased up to 20 times in comparison with the drug itself. In vivo evaluation on 22Rv1 (PSMA+) xenograft model demonstrated a good potency of the synthesized conjugate to inhibit tumor growth: the inhibition turned out to be more than 80% at a dose of 30 mg/kg. Pharmacokinetic parameters of conjugate distribution were analyzed. Also, it was found that PSMA-targeted docetaxel conjugate is less toxic than docetaxel itself, the decrease of molar acute toxicity in comparison with free docetaxel was up to 20%. Obtained conjugate PSMA-DOC is a good candidate for further expanded preclinical trials because of high antitumor activity, fewer side toxic effects and better solubility.

Enhancing the Pharmacokinetic Profile of Interleukin 2 through Site-Specific Conjugation to a Selective Small-Molecule Transthyretin Ligand

Protein drugs hold great promise as therapeutics for a wide range of diseases. Unfortunately, one of the greatest challenges to be addressed during clinical development of protein therapeutics is their short circulation half-life. Several protein conjugation strategies have been developed for half-life extension. However, these strategies have limitations and there remains room for improvement. Here, we report a novel nature-inspired strategy for enhancing the in vivo half-life of proteins. Our strategy involves conjugating proteins to a hydrophilic small molecule that binds reversibly to the plasma protein, transthyretin. We show here that our strategy is effective in enhancing the pharmacokinetic and pharmacodynamic properties of human interleukin 2 in rats, potentially opening the door for more effective and safer cancer immunotherapies. To our knowledge, this is the first example of successful use of a small-molecule that not only extends the half-life but also maintains the smaller size, binding potency, and hydrophilicity of proteins.

Targeted Delivery of Cabazitaxel Using Cyclic Cell-Penetrating Peptide and Biomarkers of Extracellular Matrix for Prostate and Breast Cancer Therapy

Targeted drug delivery for cancer therapy is an emerging area of research. Cancer cells overexpress certain biomarkers that can be exploited for their targeted therapy. Cyclic cell-penetrating peptides (cCPP) are increasingly assessed for intracellular cargo delivery in cancer cells. In this study, we have conjugated cabazitaxel (CBT) to the cCPP via an ester bond to assist CBT release in the tumor's acidic environment. Integrin targeting (RGDC, TP1) and extra domain B of fibronectin (EDB-Fn) targeting (CTVRTSAD, TP2) peptides were linked to the peptide-drug conjugate (cCPP-CBT) via a disulfide bond to provide targeting ability to the conjugates until they reach the tumor site. Conjugate 11 (TP1-cCPP-CBT) and conjugate 16 (TP2-cCPP-CBT) showed approximately 3-4-fold less antiproliferative activity on integrin and EDB-FN overexpressing cancer cell lines as compared to the CBT analogue used for comparison (CBT-GA, 5). Conjugates (11 and 16) were less toxic (31-34-fold less antiproliferative activity) to the normal human embryonic kidney (HEK-293) cells as compared to CBT. The flow cytometry and quantitative confocal microscopy data further confirm the selective efficacy of conjugates (TP1-cCPP-FAM (10) and TP1-cCPP-FAM (15)) toward biomarker overexpressing cancer cells. Furthermore, the stability and release studies of conjugate 11 revealed its therapeutic potential under different conditions, such as human plasma, different pHs, and redox conditions. This conjugation strategy was proven to enhance chemotherapeutics agents' efficacy and targeting and can be applied to other chemotherapeutic agents.

Improving the Stability of Maleimide-Thiol Conjugation for Drug Targeting

Maleimides are essential compounds for drug conjugation reactions via thiols to antibodies, peptides and other targeting units. However, one main drawback is the occurrence of thiol exchange reactions with, for example, glutathione resulting in loss of the targeting ability. A new strategy to overcome such retro-Michael exchange processes of maleimide-thiol conjugates by stabilization of the thiosuccinimide via a transcyclization reaction is presented. This reaction enables the straightforward synthesis of stable maleimide-thiol adducts essential in drug-conjugation applications.