Synthesis and Preclinical Evaluation of Urea-Based Prostate-Specific Membrane Antigen-Targeted Conjugates Labeled with 177Lu.

177Lu-labeled small-molecule prostate-specific membrane antigen (PSMA) targeted tracers are therapeutic agents for metastatic castration-resistant prostate cancer. Optimizing molecular design holds the potential to further enhance the pharmacokinetic properties of PSMA-targeted agents while preserving their potent therapeutic effects. In this study, six novel N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-(S)-l-lysine (DCL) urea-based PSMA ligand 2,2',2″,2‴-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid conjugates were synthesized. These conjugates feature polypeptide linkers containing the Phe-Phe peptide sequence and an aromatic fragment at the ε-NH-Lys group of the DCL fragment. The synthesis yielded products with satisfactory yields ranging from 60% to 72%, paving the way for their preclinical evaluation. The labeling of the new variants of urea-based PSMA inhibitors provided a radiochemical yield of over 95%. The 177Lu-labeled conjugates demonstrated specific and moderate affinity binding to PSMA-expressing human cancer cells PC3-pip in vitro and specific accumulation in PSMA-expressing xenografts in vivo. Based on the results, both the lipophilicity and the type of substituent in the linker significantly influence the binding properties of the PSMA inhibitor and its biodistribution profile. Specifically, the studied variants containing a bromine substituent or a hydroxyl group introduced into the aromatic fragment of the phenylalanyl residue in DCL exhibit higher affinities to PSMA compared to variants with only a chlorine-substituted aromatic fragment or variants without any substituents. The [177Lu]Lu-13C with the bromine substituent was characterized by the highest activity accumulation in blood, salivary glands, muscle, bone, and gastrointestinal tract and had inasmuch as an unfavorable pharmacokinetic profile. The negative charge of the carboxyl group in the phenyl moiety of the [177Lu]Lu-13A variant has demonstrated a positive effect on reducing the retention of activity in the liver and the kidneys (the ratio of tumor to kidneys was 1.3-fold). Low accumulation in normal tissues in vivo indicates that this novel PSMA-targeting inhibitor is a promising radioligand.

Fluorescent Conjugates Based on Prostate-Specific Membrane Antigen Ligands as an Effective Visualization Tool for Prostate Cancer.

Fluorescent dyes are widely used in histological studies and in intraoperative imaging, including surgical treatment of prostate cancer (PC), which is one of the most common types of cancerous tumors among men today. Targeted delivery of fluorescent conjugates greatly improves diagnostic efficiency and allows for timely correct diagnosis. In the case of PC, the protein marker is a prostate-specific membrane antigen (PSMA). To date, a large number of diagnostic conjugates targeting PSMA have been created based on modified urea. The review focuses on the conjugates selectively binding to PSMA and answers the following questions: What fluorescent dyes are already in use in the field of PC diagnosis? What factors influence the structure-activity ratio of the final molecule? What features should be considered when selecting a fluorescent tag to create new diagnostic conjugates? And what could be suggested to further development in this field at the present time?

Synthesis, 123I-Radiolabeling Optimization, and Initial Preclinical Evaluation of Novel Urea-Based PSMA Inhibitors with a Tributylstannyl Prosthetic Group in Their Structures.

Prostate-specific membrane antigen (PSMA) has been identified as a target for the development of theranostic agents. In our current work, we describe the design and synthesis of novel N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-(S)-L-lysine (DCL) urea-based PSMA inhibitors with a chlorine-substituted aromatic fragment at the lysine ε-nitrogen atom, a dipeptide including two phenylalanine residues in the L-configuration as the peptide fragment of the linker, and 3- or 4-(tributylstannyl)benzoic acid as a prosthetic group in their structures for radiolabeling. The standard compounds [127I]PSMA-m-IB and [127I]PSMA-p-IB for comparative and characterization studies were first synthesized using two alternative synthetic approaches. An important advantage of the alternative synthetic approach, in which the prosthetic group (NHS-activated esters of compounds) is first conjugated with the polypeptide sequence followed by replacement of the Sn(Bu)3 group with radioiodine, is that the radionuclide is introduced in the final step of synthesis, thereby minimizing operating time with iodine-123 during the radiolabeling process. The obtained DCL urea-based PSMA inhibitors were radiolabeled with iodine-123. The radiolabeling optimization results showed that the radiochemical yield of [123I]PSMA-p-IB was higher than that of [123I]PSMA-m-IB, which were 74.9 ± 1.0% and 49.4 ± 1.2%, respectively. The radiochemical purity of [123I]PSMA-p-IB after purification was greater than 99.50%. The initial preclinical evaluation of [123I]PSMA-p-IB demonstrated a considerable affinity and specific binding to PC-3 PIP (PSMA-expressing cells) in vitro. The in vivo biodistribution of this new radioligand [123I]PSMA-p-IB showed less accumulation than [177Lu]Lu-PSMA-617 in several normal organs (liver, kidney, and bone). These results warrant further preclinical development, including toxicology evaluation and experiments in tumor-bearing mice.

Synthesis of Prostate-Specific Membrane Antigen-Targeted Bimodal Conjugates of Cytotoxic Agents and Antiandrogens and Their Comparative Assessment with Monoconjugates.

Prostate cancer is the second most common cancer among men. We designed and synthesized new ligands targeting prostate-specific membrane antigen and suitable for bimodal conjugates with diagnostic and therapeutic agents. In vitro studies of the affinity of the synthesized compounds to the protein target have been carried out. Based on these ligands, a series of bimodal conjugates with a combination of different mitosis inhibitors and antiandrogenic drugs were synthesized. The cytotoxicity of the compounds obtained in vitro was investigated on three different cell lines. The efficacy of the two obtained conjugates was evaluated in vivo in xenograft models of prostate cancer. These compounds have been shown to be highly effective in inhibiting the growth of PSMA-expressing tumors.

Synthesis and Preclinical Evaluation of Small-Molecule Prostate-Specific Membrane Antigen-Targeted Abiraterone Conjugate.

Prostate cancer is the second most common type of cancer among men. The main method of its treatment is androgen deprivation therapy, which has a wide range of side effects. One of the solutions to this challenge is the targeted delivery of drugs to prostate cancer cells. In this study, we performed the synthesis of a novel small-molecule PSMA-targeted conjugate based on abiraterone. Cytotoxicity, the induction of intracellular reactive oxygen species, and P450-cytochrome species inhibition were investigated for this conjugate PSMA-abiraterone. The conjugate demonstrated a preferential effect on prostate tumor cells, remaining inactive at up to 100 µM in human fibroblast cells. In addition, it revealed preferential efficacy, specifically on PSMA-expressing lines with a 65% tumor growth inhibition level on 22Rv1 (PSMA+) xenografts after 14-fold oral administration of PSMA-Abi at a single dose of 500 mg/kg (7.0 g/kg total dose) was observed. This compound showed significantly reduced acute toxicity with comparable efficacy compared to AbiAc.

Synthesis and initial in vitro evaluation of PSMA-targeting ligands with a modified aromatic moiety at the lysine ε-nitrogen atom.

We report an improved series of ligands targeting prostate specific membrane antigen (PSMA). The new compounds were designed by the introduction of changes in the structure of the aromatic fragment at ε-nitrogen atom of lysine that resulted in improved biological parameters. Some of them demonstrated high selectivity and nanomolar IC50 values. We synthesized and tested two conjugates with a fluorescent label Sulfo-Cy5 as an example of the use of the obtained PSMA inhibitors as a basis for the creation of diagnostic preparations.

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.

The Importance of Linkers in the Structure of PSMA Ligands.

Cancer is one of the leading social problems of the modern world. Today prostate cancer is the second leading cause of cancer deaths among men. Targeted drug delivery is widely used to treat and diagnose prostate cancer. Conjugates selectively binding to prostatespecific membrane antigen-based on urea ligands are being actively developed against this disease. The linker has a significant influence on the biological activity of such conjugates. The linker performs a large number of functions, and its modification is one of the key methods for creating the best pharmacological profile. This review aims to discuss and analyze the main approaches to the method of introduction and synthesis of linkers for this type of conjugates without a description of the influence of biologically active molecules, as well as to establish the key modification methods that have a significant role on the structure-activity relationship. For this purpose, a review of the current scientific literature was performed, both for the conjugates under development and those already undergoing clinical trials. It was found that the optimal structure is a linker containing an aliphatic fragment near the vector- molecule (n(CH2) = 3-6), followed by a polypeptide chain consisting of 2 to 4 amino acid residues. The presence of a Phe-Phe dipeptide chain or the introduction of negatively charged groups also has a positive effect. Ongoing research in this field helps to establish the accurate effect of each linker fragment, and the development of solid-phase synthesis methods makes it much easier to achieve this goal.

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 low-molecular double conjugates for diagnostics and therapy.

This review presents data on dual conjugates of therapeutic and diagnostic action for targeted delivery to prostate cancer cells. The works of the last ten years on this topic were analyzed. The mail attention focuses on low-molecular-weight conjugates directed to the prostate-specific membrane antigen (PSMA); the comparison of high and low molecular weight PSMA-targeted conjugates was made. The considered conjugates were divided in the review into two main classes: diagnostic bimodal conjugates (which are containing two fragments for different types of diagnostics), theranostic conjugates (containing both therapeutic and diagnostic agents); also bimodal high molecular weight therapeutic conjugates containing two therapeutic agents are briefly discussed. The data of in vitro and in vivo studies for PSMA-targeted double conjugates available by the beginning of 2021 have been analyzed.

Synthesis and Biological Evaluation of PSMA Ligands with Aromatic Residues and Fluorescent Conjugates Based on Them.

Prostate-specific membrane antigen (PSMA), also known as glutamate carboxypeptidase II (GCPII), is a suitable target for specific delivery of antitumor drugs and diagnostic agents due to its overexpression in prostate cancer cells. In the current work, we describe the design, synthesis, and biological evaluation of novel low-molecular PSMA ligands and conjugates with fluorescent dyes FAM-5, SulfoCy5, and SulfoCy7. In vitro evaluation of synthesized PSMA ligands on the activity of PSMA shows that the addition of aromatic amino acids into a linker structure leads to a significant increase in inhibition. The conjugates of the most potent ligand with FAM-5 as well as SulfoCy5 demonstrated high affinities to PSMA-expressing tumor cells in vitro. In vivo biodistribution in 22Rv1 xenografts in Balb/c nude mice of PSMA-SulfoCy5 and PSMA-SulfoCy7 conjugates with a novel PSMA ligand demonstrated good visualization of PSMA-expressing tumors. Also, the conjugate PSMA-SulfoCy7 demonstrated the absence of any explicit toxicity up to 87.9 mg/kg.

Polypeptide-Based Molecular Platform and Its Docetaxel/Sulfo-Cy5-Containing Conjugate for Targeted Delivery to Prostate Specific Membrane Antigen.

A strategy for stereoselective synthesis of molecular platform for targeted delivery of bimodal therapeutic or theranostic agents to the prostate-specific membrane antigen (PSMA) receptor was developed. The proposed platform contains a urea-based, PSMA-targeting Glu-Urea-Lys (EuK) fragment as a vector moiety and tripeptide linker with terminal amide and azide groups for subsequent addition of two different therapeutic and diagnostic agents. The optimal method for this molecular platform synthesis includes (a) solid-phase assembly of the polypeptide linker, (b) coupling of this linker with the vector fragment, (c) attachment of 3-aminopropylazide, and (d) amide and carboxylic groups deprotection. A bimodal theranostic conjugate of the proposed platform with a cytostatic drug (docetaxel) and a fluorescent label (Sulfo-Cy5) was synthesized to demonstrate its possible sequential conjugation with different functional molecules.