Selective Tumor Targeting of Desacetyl Vinblastine Hydrazide and Tubulysin B via Conjugation to a Cholecystokinin 2 Receptor (CCK2R) Ligand

Small Molecule-Drug Conjugates: Opportunities for the Development of Targeted Anticancer Drugs

Conventional chemotherapy is insufficient for precise cancer treatment due to its lack of selectivity and inevitable side effects. Targeted drugs have emerged as a promising solution for precise cancer treatment. A common strategy is to conjugate therapeutic agents with ligands that can specifically bind to tumor cells, providing targeted therapy. Similar to the more successful antibody drug conjugates (ADCs), small molecule drug conjugates (SMDCs) are another promising class of targeted drugs, consisting of three parts: targeting ligand, cleavable linker and payload. Compared to ADCs, SMDCs have the advantages of smaller size, better permeability, simpler preparation process and non-immunogenicity, making them a promising alternative to ADCs. This review describes the characteristics of the targeting ligand, linker and payload of SMDCs and the criteria for selecting a suitable one. We also discuss recently reported SMDCs and list some successful SMDCs that have entered clinical trials.

Synthetic Transformation of 2-{2-Fluoro[1,1'-biphenyl]-4-yl} Propanoic Acid into Hydrazide-Hydrazone Derivatives: In Vitro Urease Inhibition and In Silico Study

In the present study, 28 acyl hydrazones (4-31) of flurbiprofen were synthesized in good to excellent yield by reacting different aromatic aldehydes with the commercially available drug flurbiprofen. The compounds were deduced with the help of different spectroscopic techniques like ¹H-NMR and HREI-MS and finally evaluated for in vitro urease inhibitory activity. All of the synthesized products demonstrated good inhibitory activities in the range of IC₅₀ = 18.92 ± 0.61 to 90.75 ± 7.71 μM as compared to standard thiourea (IC₅₀ = 21.14 ± 0.42 μM). Compound 30 was found to be the most active among the series better than the standard thiourea. A structure-activity relationship (SAR) study revealed that the presence of electron-donating groups on the phenyl ring plays a prominent role in the inhibition of the urease enzyme. Moreover, in silico molecular modeling analysis was carried out to study the effect of substituents in synthesized derivatives on the binding interactions with the urease enzyme.

Design, Synthesis, Crystal Structure, In Vitro and In Silico Evaluation of New N'-Benzylidene-4-tert-butylbenzohydrazide Derivatives as Potent Urease Inhibitors

Hydrazides play a vital role in making biologically active compounds in various fields of chemistry. These determine antioxidant, antidepressant, antimalarial, anti-inflammatory, antiglycating, and antimicrobial activity. In the present study, twenty-three new N' benzylidene-4-(tert-butyl)benzohydrazide derivatives (4-26) were synthesized by the condensation of aromatic aldehydes and commercially available 4-(tert-butyl)benzoic acid. All the target compounds were successfully synthesized from good to excellent yield; all synthesized derivatives were characterized via spectroscopic techniques such as HREI-MS and ¹H-NMR. Synthesized compounds were evaluated for in vitro urease inhibition. All synthesized derivatives demonstrated good inhibitory activities in the range of IC₅₀ = 13.33 ± 0.58-251.74 ± 6.82 µM as compared with standard thiourea having IC₅₀ = 21.14 ± 0.425 µM. Two compounds, 6 and 25, were found to be more active than standard. SAR revealed that electron donating groups in phenyl ring have more influence on enzyme inhibition. However, to gain insight into the participation of different substituents in synthesized derivatives on the binding interactions with urease enzyme, in silico (computer simulation) molecular modeling analysis was carried out.

Nonpeptidic Z360-Analogs Tagged with Trivalent Radiometals as Anti-CCK2R Cancer Theranostic Agents: A Preclinical Study

(1) Background: Theranostic approaches in the management of cholecystokinin subtype 2 receptor (CCK₂R)-positive tumors include radiolabeled gastrin and CCK motifs. Moving toward antagonist-based CCK₂R-radioligands instead, we herein present three analogs of the nonpeptidic CCK₂R-antagonist Z360, GAS1/2/3. Each was conjugated to a different chelator (DOTA, NODAGA or DOTAGA) for labeling with medically relevant trivalent radiometals (e.g., Ga-68, In-111, Lu-177) for potential use as anti-CCK₂R cancer agents; (2) Methods: The in vitro properties of the thee analogs were compared in stably transfected HEK293-CCK₂R cells. Biodistribution profiles were compared in SCID mice bearing twin HEK293-CCK₂R and wtHEK293 tumors; (3) Results: The GAS1/2/3 analogs displayed high CCK₂R-affinity (lower nM-range). The radioligands were fairly stable in vivo and selectively targeted the HEK293-CCK₂R, but not the CCK₂R-negative wtHEK293 tumors in mice. Their overall pharmacokinetic profile was found strongly dependent on the radiometal-chelate. Results could be visualized by SPECT/CT for the [¹¹¹In]In-analogs; (4) Conclusions: The present study highlighted the high impact of the radiometal-chelate on the end-pharmacokinetics of a new series of Z360-based radioligands, revealing candidates with promising properties for clinical translation. It also provided the impetus for the development of a new class of nonpeptidic radioligands for CCK₂R-targeted theranostics of human cancer.

An ultra-high-affinity small organic ligand of fibroblast activation protein for tumor-targeting applications

We describe the development of OncoFAP, an ultra-high-affinity ligand of fibroblast activation protein (FAP) for targeting applications with pan-tumoral potential. OncoFAP binds to human FAP with affinity in the subnanomolar concentration range and cross-reacts with the murine isoform of the protein. We generated various fluorescent and radiolabeled derivatives of OncoFAP in order to study biodistribution properties and tumor-targeting performance in preclinical models. Fluorescent derivatives selectively localized in FAP-positive tumors implanted in nude mice with a rapid and homogeneous penetration within the neoplastic tissue. Quantitative in vivo biodistribution studies with a lutetium-177-labeled derivative of OncoFAP revealed a preferential localization in tumors at doses of up to 1,000 nmol/kg. More than 30% of the injected dose had already accumulated in 1 g of tumor 10 min after intravenous injection and persisted for at least 3 h with excellent tumor-to-organ ratios. OncoFAP also served as a modular component for the generation of nonradioactive therapeutic products. A fluorescein conjugate mediated a potent and FAP-dependent tumor cell killing activity in combination with chimeric antigen receptor (CAR) T cells specific to fluorescein. Similarly, a conjugate of OncoFAP with the monomethyl auristatin E-based Vedotin payload was well tolerated and cured tumor-bearing mice in combination with a clinical-stage antibody-interleukin-2 fusion. Collectively, these data support the development of OncoFAP-based products for tumor-targeting applications in patients with cancer.

Small molecule drug conjugates (SMDCs): an emerging strategy for anticancer drug design and discovery

Mechanisms of how SMDCs work. Small molecule drugs are conjugated with the targeted ligand using pH sensitive linkers which allow the drug molecule to get released at lower lysosomal pH. It helps to accumulate the chemotherapeutic agents to be localized in the tumor environment upon cleaving of the pH-labile bonds.

Radiolabelled CCK2 R Antagonists Containing PEG Linkers: Design, Synthesis and Evaluation

The cholecystokinin-2/gastrin receptor (CCK₂ R) is considered a suitable target for the development of radiolabelled antagonists, due to its overexpression in various tumours, but no such compounds are available in clinical use. Therefore, we designed novel 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-conjugated ligands based on CCK₂ R antagonist Z360/nastorazepide. As a proof of concept that CCK₂ R antagonistic activity can be retained by extending the Z360/nastorazepide structure using suitable linker, we present herein three compounds containing various PEG linkers synthesised on solid phase and in solution. The antagonistic properties were measured in a functional assay in the A431-CCK₂ R cell line (in the presence of agonist G17), with IC₅₀ values of 3.31, 4.11 and 10.4 nM for compounds containing PEG₄ , PEG₆ and PEG₁₂ , respectively. All compounds were successfully radiolabelled with indium-111, lutetium-177 and gallium-68 (incorporation of radiometal >95 %). The gallium-68-labelled compounds were stable for up to 2 h (PBS, 37 °C). log D_7.4 values were determined for indium-111- and gallium-68-labelled compounds, showing improved hydrophilicity compared to the reference compound.

[99mTc]Tc-DGA1, a Promising CCK2R-Antagonist-Based Tracer for Tumor Diagnosis with Single-Photon Emission Computed Tomography

Radiolabeled gastrin analogues have been proposed for theranostics of cholecystokinin subtype 2 receptor (CCK₂R)-positive cancer. Peptide radioligands based on other receptor antagonists have displayed superior pharmacokinetics and higher biosafety than agonists. Here, we present DGA1, a derivative of the nonpeptidic CCK₂R antagonist Z-360 carrying an acyclic tetraamine, for [^99mTc]Tc labeling. Preclinical comparison of [^99mTc]Tc-DGA1 with [^99mTc]Tc-DG2 (CCK₂R-agonist reference) was conducted in HEK293-CCK₂R/CCK_2i4svR cells and mice models, qualifying [^99mTc]Tc-DGA1 for further study in patients with CCK₂R-positive tumors and single-photon emission computed tomography/CT.

Design and validation of fibroblast activation protein alpha targeted imaging and therapeutic agents

Background: Cancer-associated fibroblasts (CAFs) comprise a major cell type in the tumor microenvironment where they support tumor growth and survival by producing extracellular matrix, secreting immunosuppressive cytokines, releasing growth factors, and facilitating metastases. Because tumors with elevated CAFs are characterized by poorer prognosis, considerable effort is focused on developing methods to quantitate, suppress and/or eliminate CAFs. We exploit the elevated expression of fibroblast activation protein (FAP) on CAFs to target imaging and therapeutic agents selectively to these fibroblasts in solid tumors. Methods: FAP-targeted optical imaging, radioimaging, and chemotherapeutic agents were synthesized by conjugating FAP ligand (FL) to either a fluorescent dye, technetium-99m, or tubulysin B hydrazide. In vitro and in vivo studies were performed to determine the specificity and selectivity of each conjugate for FAP in vitro and in vivo. Results: FAP-targeted imaging and therapeutic conjugates showed high binding specificity and affinity in the low nanomolar range. Injection of FAP-targeted 99mTc into tumor-bearing mice enabled facile detection of tumor xenografts with little off-target uptake. Optical imaging of malignant lesions was also readily achieved following intravenous injection of FAP-targeted near-infrared fluorescent dye. Finally, systemic administration of a tubulysin B conjugate of FL promoted complete eradication of solid tumors with no evidence of gross toxicity to the animals. Conclusion: In view of the near absence of FAP on healthy cells, we conclude that targeting of FAP on cancer-associated fibroblasts can enable highly specific imaging and therapy of solid tumors.

The nuclear export inhibitor aminoratjadone is a potent effector in extracellular-targeted drug conjugates

The concept of targeted drug conjugates has been successfully translated to clinical practice in oncology. Whereas the majority of cytotoxic effectors in drug conjugates are directed against either DNA or tubulin, our study aimed to validate nuclear export inhibition as a novel effector principle in drug conjugates. For this purpose, a semisynthetic route starting from the natural product ratjadone A, a potent nuclear export inhibitor, has been developed. The biological evaluation of ratjadones functionalized at the 16-position revealed that oxo- and amino-analogues had very high potencies against cancer cell lines (e.g. 16R-aminoratjadone 16 with IC50 = 260 pM against MCF-7 cells, or 19-oxoratjadone 14 with IC50 = 100 pM against A-549 cells). Mechanistically, the conjugates retained a nuclear export inhibitory activity through binding CRM1. To demonstrate a proof-of-principle for cellular targeting, folate- and luteinizing hormone releasing hormone (LHRH)-based carrier molecules were synthesized and coupled to aminoratjadones as well as fluorescein for cellular efficacy and imaging studies, respectively. The Trojan-Horse conjugates selectively addressed receptor-positive cell lines and were highly potent inhibitors of their proliferation. For example, the folate conjugate FA-7-Val-Cit-pABA-16R-aminoratjadone had an IC50 of 34.3 nM, and the LHRH conjugate d-Orn-Gose-Val-Cit-pABA-16R-aminoratjadone had an IC50 of 12.8 nM. The results demonstrate that nuclear export inhibition is a promising mode-of-action for extracellular-targeted drug conjugate payloads.

Small molecule targeted NIR dye conjugate for imaging LHRH receptor positive cancers

Overexpression of Luteinizing Hormone Releasing Hormone Receptor (LHRH-R) in various cancers and restricted expression of the receptor in healthy cells qualifies it as a valuable cancer biomarker. Previously, LHRH-R targeted peptides have been utilized to deliver attached payloads to LHRH-R expressing cancers. We report here for the first time the utilization of a small molecule non-peptidic ligand (BOEPL) of LHRH-R to deliver attached payloads to LHRH-R positive tumors. For this purpose, we linked the BOEPL ligand to a near infrared dye via various linkers. In vitro, these conjugates demonstrated low nanomolar binding affinity and in vivo they exhibited receptor-mediated uptake specifically in tumor tissue. Moreover, tumor uptake could be blocked by administration of excess unlabeled conjugate, and time course experiments showed retention of the dye conjugate in the tumor up to 12 h post injection. Because uptake of BOEPL-targeted NIR dye conjugates by nonmalignant organs/tissues was negligible and since the transient presence of targeted NIR dye in the kidneys was a result of clearance mechanism, we suggest that a BOEPL-targeted NIR dye might constitute a useful agent for fluorescence-guided surgery of LHRH-R positive cancers. Moreover, our results also provide proof of concept that BOEPL can be successfully used to deliver attached payloads to LHRH-R positive tumors in vivo.

In Vivo Evaluation of Ligand Targeted Drug Conjugates for Cancer Therapy

The development of small molecule ligand-targeted therapeutics is currently of paramount importance for treatment of cancer due to their potential to reduce system toxicity and increase potency of a delivered chemotherapeutic drug. The main aim of a targeted drug-delivery technique is to release the drug cargo selectively into tumor tissues, avoiding off-site toxicity to healthy tissues and organs during chemotherapy. In this strategy, a chemotherapeutic drug is conjugated to a homing ligand, which has high affinity for proteins over-expressed on cancer cells, via a peptide linker and a self-immolative segment that facilitates intracellular release of drug cargo. During development of targeted drug conjugates, preclinical evaluation in tumor models of small animals like mice adds valuable data on the clinical performance of the drug. This article contains a set of protocols for implantation of tumor, determination of optimum dosage required for effective treatment, and estimation of maximum tolerated dose required for any visible side effects during treatment of cancer in tumor models of mice. © 2018 by John Wiley & Sons, Inc.

Preparation of Ligand-Targeted Drug Conjugates for Cancer Therapy and Their Evaluation In Vitro

Present treatment strategies focus on minimizing unwanted toxicity to healthy cells during chemotherapeutic treatment. This is achieved by developing strategies to selectively deliver drugs to malignant cells over-expressing specific protein biomarkers. The drugs are attached via a self-immolative linker to a small molecule homing ligand having affinity for protein biomarkers over-expressed during disease states. Several such targeting-ligand drug conjugates have now reached preclinical and clinical trials, and this article aims to show a general methodology to prepare the same. Using solid-phase peptide synthesis (SPPS) methodology, the targeting ligand is covalently linked to a peptide spacer having appropriate hydrophobic and hydrophilic amino acids. The targeting ligand-attached peptide spacer is next conjugated with the required drug molecule through a cleavable disulfide bond in a solution-phase reaction. This protocol further elucidates the step-by-step procedures to be followed for complete evaluation of newly synthesized ligand-targeted drug conjugates in vitro. © 2018 by John Wiley & Sons, Inc.

Structural recognition of tubulysin B derivatives by multidrug resistance efflux transporters in human cancer cells

Multidrug resistance (MDR) is a major hindrance to curative chemotherapy of various human malignancies. Hence, novel chemotherapeutics must be evaluated for their recognition by MDR efflux transporters. Herein we explored the cytotoxic activity of synthetic tubulysin B (Tub-B, EC1009) derivatives (Tub-B-hydrazide/EC0347 and Tub-B bis-ether/EC1820), and their recognition by the MDR efflux transporters P-glycoprotein 1 (P-gp), multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP). Originally isolated from Myxobacteria, tubulysins exhibited potent cytotoxic activity via microtubule depolymerization, and evaded recognition by these MDR efflux pumps. We show that subtle modifications in the natural Tub-B structure enhance its cytotoxicity and drug efflux efficiency. Whereas increasing the lipophilicity of Tub-B drugs enhanced their diffusion into the cell and consequently decreased the IC₅₀ values (≥ 0.27 nM), increasing drug polarity enhanced their recognition by P-gp (>200-fold resistance in P-gp-overexpressing cells). Furthermore, restricting drug exposure time to the clinically relevant 4 h pulse, markedly enhanced efflux by P-gp, resulting in a 1000-fold increased resistance, which was further enhanced upon increased P-gp levels (i.e. an additional 3-fold increase in P-gp levels resulted in >6,000-fold resistance). The unique ability of EC1009 to evade recognition by MDR efflux pumps warrants drug development of tubulysin B derivatives as potent antitumor agents which overcome MDR in cancer.

A tumor multicomponent targeting chemoimmune drug delivery system for reprograming the tumor microenvironment and personalized cancer therapy

Nanoparticle library engineered with tunable size, shape, and geometry will provide a better idea of targeting multicomponent of tumor microenvironment consisting of epithelial cells, tumor hypoxia, tumor immune cells and angiogenic blood vessels.

Copper-Free 'Click' Chemistry-Based Synthesis and Characterization of Carbonic Anhydrase-IX Anchored Albumin-Paclitaxel Nanoparticles for Targeting Tumor Hypoxia

Triple negative breast cancer (TNBC) is a difficult to treat disease due to the absence of the three unique receptors estrogen, progesterone and herceptin-2 (HER-2). To improve the current therapy and overcome the resistance of TNBC, there is unmet need to develop an effective targeted therapy. In this regard, one of the logical and economical approaches is to develop a tumor hypoxia-targeting drug formulation platform for selective delivery of payload to the drug-resistant and invasive cell population of TNBC tumors. Toward this, we developed a Carbonic Anhydrase IX (CA IX) receptor targeting human serum albumin (HSA) carriers to deliver the potent anticancer drug, Paclitaxel (PTX). We used Acetazolamide (ATZ), a small molecule ligand of CA IX to selectively deliver HSA-PTX in TNBC cells. A novel method of synthesis involving copper free ‘click’ chemistry (Dibenzocyclooctyl, DBCO) moiety with an azide-labeled reaction partner, known as Strain-Promoted Alkyne Azide Cycloaddition (SPAAC) along with a desolvation method for PTX loading were used in the present study to arrive at the CA IX selective nano-carriers, HSA-PTX-ATZ. The anticancer effect of HSA-PTX-ATZ is higher compared to HSA, PTX and non-targeted HSA-PTX in MDA-MB-231 and MDA-MB-468 cells. The cell killing effect is associated with induction of early and late phases of apoptosis. Overall, our proof-of-concept study shows a promising avenue for hypoxia-targeted drug delivery that can be adapted to several types of cancers.

Synthesis and Superpotent Anticancer Activity of Tubulysins Carrying Non-hydrolysable N-Substituents on Tubuvaline

Synthetic tubulysins 24 a-m, containing non-hydrolysable N-substituents on tubuvaline (Tuv), were obtained in high purity and good overall yields using a multistep synthesis. A key step was the formation of differently N-substituted Ile-Tuv fragments 10 by using an aza-Michael reaction of azido-Ile derivatives 8 with the α,β-unsaturated oxo-thiazole 5. A structure-activity relationship study using a panel of human tumour cell lines showed strong anti-proliferative activity for all compounds 24 a-m, with IC₅₀ values in the sub-nanomolar range, which were distinctly lower than those of tubulysin A, vinorelbine and paclitaxel. Furthermore, 24 a-m were able to overcome cross-resistance to paclitaxel and vinorelbine in two tumour cell lines with acquired resistance to doxorubicin. Compounds 24 e and 24 g were selected as leads to evaluate their mechanism of action. In vitro assays showed that both 24 e and 24 g interfere with tubulin polymerization in a vinca alkaloid-like manner and prevent paclitaxel-induced assembly of tubulin polymers. Both compounds exerted antimitotic activity and induced apoptosis in cancer cells at very low concentrations. Compound 24 e also exhibited potent antitumor activity at well tolerated doses on in vivo models of diffuse malignant peritoneal mesothelioma, such as MESOII peritoneal mesothelioma xenografts, the growth of which was not significantly affected by vinorelbine. These results indicate that synthetic tubulysins 24 could be used as standalone chemotherapeutic agents in difficult-to-treat cancers.

Ligand-decorated click polypeptide derived nanoparticles for targeted drug delivery applications

A ligand decorated, synthetic polypeptide block copolymer platform with environment-responsive capabilities was designed. We evaluated the potential of this system to function as a polymersome for targeted-delivery of a systemic chemotherapy to tumors. Our system employed click chemistry to provide a pH-responsive polypeptide block that drives nanoparticle assembly, and a ligand (folic acid) conjugated PEG block that targets folate-receptor over-expressing cancer cells. These nanocarriers were found to encapsulate a high loading of conventional chemotherapeutics (e.g. doxorubicin at physiological pH) and release the active therapeutic at lysosomal pH upon cellular uptake. The presence of folic acid on the nanoparticle surface facilitated their active accumulation in folate-receptor-overexpressing cancer cells (KB), compared to untargeted carriers. Folate-targeted nanoparticles loaded with doxorubicin also showed enhanced tumor accumulation in folate-receptor positive KB xenografts, resulting in the suppression of tumor growth in an in vivo hind flank xenograft mouse model.

Small Molecules for Active Targeting in Cancer

For the purpose of this review, active targeting in cancer research encompasses strategies wherein a ligand for a cell surface receptor expressed on tumor cells is used to deliver a cytotoxic or imaging cargo. This area of research is more than two decades old, but in those 20 and more years, how many receptors have been studied extensively? What kinds of the ligands are used for active targeting? Are they mostly naturally occurring molecules such as folic acid, or synthetic substances developed in campaigns for medicinal chemistry efforts? This review outlines the most important receptor or ligand combinations that have been used in active targeting to answer these questions, and therefore to address the most important one of all: is research in active targeting affording diminishing returns, or is this an area for which the potential far exceeds progress made so far?