Indazole scaffold: a generalist for marketed and clinical drugs

A review of poly(ADP-ribose)polymerase-1 (PARP1) role and its inhibitors bearing pyrazole or indazole core for cancer therapy

Cancer is a disease with a high mortality rate characterized by uncontrolled proliferation of abnormal cells. The hallmarks of cancer evidence the acquired cells characteristics that promote the growth of malignant tumours, including genomic instability and mutations, the ability to evade cellular death and the capacity of sustaining proliferative signalization. Poly(ADP-ribose) polymerase-1 (PARP1) is a protein that plays key roles in cellular regulation, namely in DNA damage repair and cell survival. The inhibition of PARP1 promotes cellular death in cells with homologous recombination deficiency, and therefore, the interest in PARP protein has been rising as a target for anticancer therapies. There are already some PARP1 inhibitors approved by Food and Drug Administration (FDA), such as Olaparib and Niraparib. The last compound presents in its structure an indazole core. In fact, pyrazoles and indazoles have been raising interest due to their various medicinal properties, namely, anticancer activity. Derivatives of these compounds have been studied as inhibitors of PARP1 and presented promising results. Therefore, this review aims to address the importance of PARP1 in cell regulation and its role in cancer. Moreover, it intends to report a comprehensive literature review of PARP1 inhibitors, containing the pyrazole and indazole scaffolds, published in the last fifteen years, focusing on structure-activity relationship aspects, thus providing important insights for the design of novel and more effective PARP1 inhibitors.

Synthesis of Indazoles via N-N Bond-Forming Oxidative Cyclization from 2-Aminomethyl-phenylamines

Herein we report a method for the synthesis of indazoles from readily available 2-aminomethyl-phenylamines via N-N bond-forming oxidative cyclization. Inspired by indazole formation initially observed as a side product by N. Coskun et al. we developed a robust protocol to access indazoles in all three tautomeric forms. The method selectively gives access to various 2-substituted 2H-indazoles which are frequently used in drug design, and we also demonstrated its applicability to less studied 3H-indazoles.

Heterocyclic Merging of Stereochemically Diverse Chiral Piperazines and Morpholines with Indazoles

We report a heterocyclic merging approach to construct novel indazolo-piperazines and indazolo-morpholines. Starting from chiral diamines and amino alcohols, novel regiochemically (1,3 and 1,4) and stereochemically diverse (relative and absolute) cohorts of indazolo-piperazines and indazolo-morpholines were obtained within six or seven steps. The key transformations involved are a Smiles rearrangement to generate the indazole core structure and a late-stage Michael addition to build the piperazine and morpholine heterocycles. We further explored additional vector diversity by incorporating substitutions on the indazole aromatic ring, generating a total of 20 unique, enantiomerically pure heterocyclic scaffolds.

Exploring new histone deacetylase 6 inhibitors and their effects on reversing the α-tubulin deacetylation and cell morphology changes caused by methamphetamine

Indazole-based HDAC6 inhibitors with novel zinc-binding modifications were synthesized and evaluated to determine their potential to inhibit HDAC6. The analogs were subjected to a histone deacetylase (HDAC) enzyme assay, which led to identification of compounds 3a and 3b. Both compounds demonstrated higher potency and selectivity as HDAC6 inhibitors with IC50 values of 9.1 nM and 9.0 nM, respectively, and highlighted the importance of the hydroxamic acid moiety for binding to Zn2+ inside the catalytic pocket of HDAC enzymes. In the neuroblastoma SH-SY5Y cell line, both compounds efficiently acetylated α-tubulin but not histone H3 at a low concentration of 0.5 µM. Moreover, compounds 3a and 3b effectively reversed the deacetylation of α-tubulin caused by methamphetamine in the SH-SY5Y cell line, suggesting the potential usefulness of HDAC6 selective inhibition in restoring blood brain barrier integrity by reversing methamphetamine-induced deacetylation.

A Scalable and Metal-Free Synthesis of Indazoles from 2-Aminophenones and In Situ Generated De-Boc-Protected O-Mesitylsulfonyl Hydroxylamine Derivatives

A one-pot metal-free protocol to access indazoles from easily available 2-aminophenones and hydroxylamine derivatives has been achieved. The reaction is operationally simple, mild, and insensitive to air and moisture. A broad range of indazoles were prepared in good to excellent yield (up to 97% yield), and the reaction displayed a broad functional group tolerance. The reaction was performed at gram scale, and its synthetic application was exhibited through the rapid and efficient preparation of bioactive molecule YC-3 and FDA-approved drug axitinib.

The Importance of the Pyrazole Scaffold in the Design of Protein Kinases Inhibitors as Targeted Anticancer Therapies

The altered activation or overexpression of protein kinases (PKs) is a major subject of research in oncology and their inhibition using small molecules, protein kinases inhibitors (PKI) is the best available option for the cure of cancer. The pyrazole ring is extensively employed in the field of medicinal chemistry and drug development strategies, playing a vital role as a fundamental framework in the structure of various PKIs. This scaffold holds major importance and is considered a privileged structure based on its synthetic accessibility, drug-like properties, and its versatile bioisosteric replacement function. It has proven to play a key role in many PKI, such as the inhibitors of Akt, Aurora kinases, MAPK, B-raf, JAK, Bcr-Abl, c-Met, PDGFR, FGFRT, and RET. Of the 74 small molecule PKI approved by the US FDA, 8 contain a pyrazole ring: Avapritinib, Asciminib, Crizotinib, Encorafenib, Erdafitinib, Pralsetinib, Pirtobrutinib, and Ruxolitinib. The focus of this review is on the importance of the unfused pyrazole ring within the clinically tested PKI and on the additional required elements of their chemical structures. Related important pyrazole fused scaffolds like indazole, pyrrolo[1,2-b]pyrazole, pyrazolo[4,3-b]pyridine, pyrazolo[1,5-a]pyrimidine, or pyrazolo[3,4-d]pyrimidine are beyond the subject of this work.

Arenes participate in 1,3-dipolar cycloaddition with in situ-generated diazoalkenes

The venerable 1,3-dipolar cycloaddition has been widely used in organic synthesis for the construction of various heterocycles. However, in its century-long history, the simple and omnipresent aromatic phenyl ring has remained a stubbornly unreactive dipolarophile. Here we report 1,3-dipolar cycloaddition between aromatic groups and diazoalkenes, generated in situ from lithium acetylides and N-sulfonyl azides. The reaction results in densely functionalized annulated cyclic sulfonamide-indazoles that can be further converted into stable organic molecules that are important in organic synthesis. The involvement of aromatic groups in the 1,3-dipolar cycloadditions broadens the synthetic utility of diazoalkenes, a family of dipoles that have been little explored so far and are otherwise difficult to access. The process described here provides a route for the synthesis of medicinally relevant heterocycles and can be extended to other arene-containing starting materials. Computational examination of the proposed reaction pathway revealed a series of finely orchestrated bond-breaking and bond-forming events that ultimately lead to the annulated products.

Design, Synthesis and Antitumor Activity of 1H-indazole-3-amine Derivatives

A series of indazole derivatives were designed and synthesized by molecular hybridization strategy, and these compounds were evaluated the inhibitory activities against human cancer cell lines of lung (A549), chronic myeloid leukemia (K562), prostate (PC-3), and hepatoma (Hep-G2) by methyl thiazolyl tetrazolium (MTT) colorimetric assay. Among these, compound 6o exhibited a promising inhibitory effect against the K562 cell line with the IC₅₀ (50% inhibition concentration) value of 5.15 µM, and this compound showed great selectivity for normal cell (HEK-293, IC₅₀ = 33.2 µM). Moreover, compound 6o was confirmed to affect apoptosis and cell cycle possibly by inhibiting Bcl2 family members and the p53/MDM2 pathway in a concentration-dependent manner. Overall, this study indicates that compound 6o could be a promising scaffold to develop an effective and low-toxic anticancer agent.

A review on synthetic strategy, molecular pharmacology of indazole derivatives, and their future perspective

With different nitrogen-containing heterocyclic moieties, Indazoles earn one of the places among the top investigated molecules in medicinal research. Indazole, an important fused aromatic heterocyclic system containing benzene and pyrazole ring with a chemical formula of C₇ H₆ N₂ , is also called benzopyrazole. Indazoles consist of three tautomeric forms in which 1H-tautomers (indazoles) and 2H-tautomers (isoindazoles) exist in all phases. The tautomerism in indazoles greatly influences synthesis, reactivity, physical and even the biological properties of indazoles. The thermodynamic internal energy calculation of these tautomers points view 1H-indazole as the predominant and stable form over 2H-indazole. The natural source of indazole is limited and exists in alkaloidal nature (i.e., nigellidine, nigeglanine, nigellicine, etc.) found from Nigella plants. Some of the FDA-approved drugs like Axitinib, Entrectinib, Niraparib, Benzydamine, and Granisetron are being used to treat renal cell cancer, non-small cell lung cancer (NSCLC), epithelial ovarian cancer, chronic inflammation, chemotherapy-induced nausea, vomiting, and many more uses. Besides all these advantages regarding its biological activity, the main issue about indazoles is the less abundance in plant sources, and their synthetic derivatives also often face problems with low yield. In this review article, we discuss its chemistry, tautomerism along with their effects, different schematics for the synthesis of indazole derivatives, and their different biological activities.

Recent Strategies in Transition-Metal-Catalyzed Sequential C–H Activation/Annulation for One-Step Construction of Functionalized Indazole Derivatives

Designing new synthetic strategies for indazoles is a prominent topic in contemporary research. The transition-metal-catalyzed C–H activation/annulation sequence has arisen as a favorable tool to construct functionalized indazole derivatives with improved tolerance in medicinal applications, functional flexibility, and structural complexity. In the current review article, we aim to outline and summarize the most common synthetic protocols to use in the synthesis of target indazoles via a transition-metal-catalyzed C–H activation/annulation sequence for the one-step synthesis of functionalized indazole derivatives. We categorized the text according to the metal salts used in the reactions. Some metal salts were used as catalysts, and others may have been used as oxidants and/or for the activation of precatalysts. The roles of some metal salts in the corresponding reaction mechanisms have not been identified. It can be expected that the current synopsis will provide accessible practical guidance to colleagues interested in the subject.

Novel indazole skeleton derivatives containing 1,2,3-triazole as potential anti-prostate cancer drugs

In this study, a novel batch of indazole containing 1,2,3-triazole agents were designed and synthesized. The antiproliferative activity of target compounds in four human cancer cells, PC-3 (human prostate cancer cell), MCF-7 (human breast cancer cell), HepG-2 (human hepatoma cell) and MGC-803 (human gastric cancer cell), was evaluated by thiazole blue (MTT). In the antiproliferative activity screening, we were surprised to find that most compounds have specific cytotoxicity to PC-3 cancer cells. In particular, 9a has an IC₅₀ value of 4.42 ± 0.06 μmol/L against PC-3 cell. Cloning experiments showed that 9a could inhibit the formation of PC-3 cancer cell clone in a dose-dependent manner. Through cell cycle arrest experiment, we found that compound 9a can block the cell cycle in G2/M phase and inhibit cell proliferation. Finally, by evaluating the safety of compound 9a, we noticed that it showed fairly good safety both in vivo and in vitro. Overall, based on the biological activity evaluation and safety, analogue 9a can be viewed as a potential lead compound for further development of novel anti-prostate cancer drug.