Cheng et al.112 in 2019 worked on the synthesis of novel derivatives as potential HDAC/BRD4 dual inhibitors and anti-leukemic agents. So, HDAC is considered as valuable target for design of anti-cancer agents having HDAC inhibitory activity. In 53, it was found that if R5 and R6 do not have any substituent and R7 consists of methyl substitution, anti-cancer activity was found to be moderate. In 52, electron donating group at R4 displayed excellent cytotoxicity only if R1, R2 and R3 do not have any substituent, but R1 has a benzyl, then activity decreases considerably. SAR studies revealed that substitution at C-5 of oxindole with methyl 50b remarkably increases the activity by 8-fold as compared to 50a suggesting that an electron donating group is favourable for the activity. Ring fluorination of 5-MeO-DMT at 4-position resulted in an increased affinity for the 5HT1A receptor, which yielded a selective and potent 4-fluoro-5-MeO-DMT (Ki 0.23 nM). In another study, this research group investigated how fluorination affects the hallucinogen-like activity of tryptamines . N-Methyl-5-tert-butyltryptamine exhibited a greater affinity toward 5-HT1D receptor, and compound 56 was the most powerful agonist with a Ki value of 0.45 nM. In a study, 5-alkhyltryptamine analogs were evaluated for finding the substituents that are important for the binding affinity of the particular molecule toward 5-HT1D receptors . The latest medicinal and synthetic chemistry-based research has concentrated on the synthesis of various types of specific ligands for 5-HTRs. The replacement at the nitrogen atom of a heterocycle must be a phenyl group, and the substitution of the group at 3-position on indolinone must be small for maintaining the activity . The branching of the sidechain can result in a reduced effect similar to the replacement of the indolinone ring. The Leimgruber–Batcho indole synthesis is an efficient method of synthesizing indole and substituted indoles. For intracellular indole detection and measurement genetically encoded indole-responsive biosensor is applicable. A number of indole derivatives have important cellular functions, including neurotransmitters such as serotonin. In 2019, Wang and colleagues48 investigated the indole-imidazole hybrid as a powerful tubulin inhibitor, resulting in two potent compounds 7a and 7b with IC50 values ranging from 1.6 to 3.7 nmol/L, which were around 2–3 times more potent than the reference drug ABI-231. The report stated that 4 (Fig. 3) possessed excellent activity at nanomolar concentration against NCI-60 human cancer cell lines with GI50 46 in 2019. The review also discusses the SAR of potential lead accountable for anticancer activity31. Motivated by the anticancer clinical drugs such as vincristine and vinblastine containing indole alkaloids, many researches have also been made in the areas of indole alkaloids from natural sources in the last three years27,28. Inspite of the crucial advances in technology and chemotherapy research, the non-selectivity to cancerous tissue and multiple-drug resistance by cancer cells continues to be a grave concern for the medicinal chemists. It is multifaceted lethal disease wherein a group of atypical body cells starts dividing uncontrollably instead of normal cell division. Due to the versatile nature of indole, it has gained immense popularity among the researchers to synthesize several compounds which encompass indole as a main nucleus and determined their various biological activities23. A series of novel cyclic bridging analogues (CBAs) of CA-4 with a phenyl or pyridine linker were developed, produced, and analysed precisely. A similar replacement as in compound 18 was also achieved with a N (Me) linker in compound 1959. Following that, it was found that the combination of a quinolinyl ring as ring A and a carbazolyl group as ring B had the highest efficacy. Small furan ring substitutions, halogenation at R1 position, and changes in furyl connectivity were the focus of synthetic modifications of 14a.
