Tubercidin is systematically named (2R,3R,4S,5R)-2-(4-aminopyrrolo[2,3-d]pyrimidin-7-yl)-5-(hydroxymethyl)oxolane-3,4-diol. It is an N-glycosylpyrrolopyrimidine ribonucleoside and a member of the 7-deazapurine class, characterized by the replacement of the nitrogen atom at position 7 of the purine ring with a carbon atom. This structural change allows tubercidin to mimic adenosine in biological systems. The 7-deaza modification makes it resistant to degradation by enzymes such as adenosine deaminase and adenosine phosphorylase, enabling it to persist longer inside cells. This stability enhances its ability to disrupt nucleic acid metabolism and other adenosine-dependent processes. This results in its potent biological activity and systemic toxicity. Other natural compounds like toyocamycin share this structural similarity, making 7-deazapurines an important class of bioactive nucleosides.[11][12]
Adenosine
Tubercidin
Toyocamycin
Occurrence
Tubercidin is naturally produced by several species of actinomycetes, particularly within the genus Streptomyces. It was first discovered in Streptomyces tubercidicus, but later identified in multiple other strains. The following species have been reported to produce tubercidin:
Due to its structural similarity to adenosine, tubercidin can interfere with various essential biological processes resultin in broad range of biological activities.
Anticancer activity
Tubercidin shows potent cytotoxic activity against various cancer cell lines, including P388 and A549 tumor cells, as well as human cancer cell lines such as HeLa, A375, and WM266.[14][22] It has shown promising anti-Small-Cell Lung Cancer (SCLC) activity both in vitro and in vivo. It selectively exhibits strong cytotoxicity against SCLC cell lines (DMS 53 and DMS 114) at low concentrations (CC50 of 0.19 µM and 0.14 µM, respectively), with minimal impact on normal bronchial cells. In SCLC xenograft mouse models, tubercidin treatment (5 mg/kg, three times a week) significantly inhibited tumor growth, with some instances of complete tumor regression.[23]The anticancer activity of tubercidin mainly arises due to the induction of apoptosis in these cells.[24] 5-Iodotubercidin, a derivative of tubercidin, has been identified as a genotoxic agent and a potent activator of the tumor suppressor protein p53, triggering DNA damage, cell cycle arrest, and necroptosis in cancer models.[25][26] Current research aims in developing less toxic derivatives of tubercidin by C6, C7, or C8 modifications on the purine ring. Additionally, new platinum(II) complexes of tubercidin are being investigated for their enhanced selectivity toward tumor cells.[22]
In parasitic organisms such as in Trypanosoma brucei, tubercidin has been found to inhibit glycolysis by targeting the enzyme phosphoglycerate kinase. Since trypanosomes rely heavily on glycolysis for energy production, this makes glycolytic enzymes attractive targets for antitrypanosomal drugs.[32]
Tubercidin has also been shown to disrupt the function of nuclear speckles (NSs), which are essential subnuclear structures enriched with RNA-binding proteins involved in mRNA splicing and processing. Upon treatment with tubercidin, poly(A)+ RNAs become dispersed and degraded across the nucleoplasm, while SC35-marked nuclear speckles remain condensed. This suggests that tubercidin selectively impairs mRNA processing without completely dismantling the speckles themselves. Under stress conditions such as hypoxia or serum starvation, this disruption can worsen cellular damage and promote apoptosis, particularly in sensitive cells like cardiomyocytes.[46]
Toxicity
Despite potent biological activities, the clinical applications of tubercidin are significantly limited due to its toxicity to mammalian cells. This manifests itself mainly as hepatotoxicity, nephrotoxicity, and cardiotoxicity.[47]
Cardiotoxicity is a notable concern with tubercidin, particularly in individuals with existing heart conditions like ischemic cardiomyopathy. Tubercidin promotes apoptosis in heart muscle cells under stress, especially in hypoxic or starved conditions. This effect appears to be linked to tubercidin's interference with nuclear speckles which are important for processing mRNA and regulating gene activity. By disrupting these functions, tubercidin may worsen damage in already weakened heart tissue.[24][47]
Hepatotoxicity and nephrotoxicity have been observed in vivo in mice and in vitro with human bone marrow progenitor cells. In mice, intravenous doses of 45 mg/kg caused high mortality rates, mainly due to liver damage. Kidney injury was also noted at higher doses. Co-administering nucleoside transport inhibitors like nitrobenzylthioinosine 5'-monophosphate (NBMPR-P) helped reduce liver toxicity by changing how tubercidin is distributed in the body. However, at high doses, NBMPR-P increased the risk of kidney damage.[47][48]
Clinical use and derivatives
Early Phase I clinical trials involving direct intravenous administration of tubercidin in humans found the drug to be unsuitable due to significant toxicity. Reported side effects included hepatic toxicity, renal toxicity like proteinuria and uremia, and hematological toxicity like venous thrombosis and leukopenia. These toxic effects have been a major barrier to the clinical use of tubercidin.[47][49][50][51]
The bioactivity of tubercidin along with its toxicity has spurred extensive exploration into its derivatives particularly with modifications at C6, C7 and C8. Some of the key ones are:
3'-deoxytubercidin: It is an antitrypanosomal agent with no toxicity at therapeutic doses.[38]
Modification of the ribose ring has also yielded results in the form of MK-608 (7-deaza-2’-C-methyladenosine), though it was ultimately unsuccessful in human clinical trial.[55]
↑ E, Mihich; CL, Simpson; AI, Mulhern (1969). "Comparative study of the toxicologic effects of 7-deazaadenosine (tubercidin) and 7-deazainosine". Cancer Research. 29 (1). Cancer Res: 116–123. ISSN0008-5472. PMID5763972.
↑ Ogbunude, P. O.; Ikediobi, C. O. (1982-09-01). "Effect of nitrobenzylthioinosinate on the toxicity of tubercidin and ethidium against Trypanosoma gambiense". Acta Tropica. 39 (3): 219–224. ISSN0001-706X. PMID6128890.
↑ Anzai, K.; Nakamura, G.; Suzuki, S. (1957-09-01). "A new antibiotic, tubercidin". The Journal of Antibiotics. 10 (5): 201–204. ISSN0021-8820. PMID13513512.
↑ Bloch, A.; Mihich, E.; Leonard, R. J.; Nichol, C. A. (1969-01-01). "Studies on the biologic activity and mode of action of 7-deazainosine". Cancer Research. 29 (1): 110–115. ISSN0008-5472. PMID4974300.
↑ Grage, T. B.; Rochlin, D. B.; Weiss, A. J.; Wilson, W. L. (1970-01-01). "Clinical studies with tubercidin administered after absorption into human erythrocytes". Cancer Research. 30 (1): 79–81. ISSN0008-5472. PMID4985935.
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