Thymidine and endogenous nucleoside substrates

Thymidine and endogenous nucleoside substrates are naturally occurring molecules, such as adenosine, cytidine, guanosine, uridine, and thymidine, that serve as the essential building blocks for DNA and RNA synthesis (PubChem) [1]. While they are not therapeutic targets in the traditional sense, such as a specific receptor or enzyme, they represent the physiological substrates for a wide array of proteins that are critical in medical therapy (StatPearls) [2]. In the nucleotide salvage pathway, enzymes like thymidine kinase and thymidylate synthase utilize these substrates to maintain the cellular pools of deoxyribonucleotides required for DNA replication and repair (UniProt) [3, 4]. In oncology, many chemotherapy agents known as antimetabolites are designed as structural analogs of these nucleosides (NCBI) [5]. These drugs exert their therapeutic effect by competing with the endogenous substrates for binding to enzymes or for incorporation into growing DNA strands, ultimately inducing cell cycle arrest and apoptosis (PubMed) [6]. Additionally, the transport of these substrates across cell membranes by proteins like the equilibrative nucleoside transporters (ENTs) is a key determinant of the intracellular concentration and efficacy of both endogenous nucleosides and their pharmacological mimics (UniProt) [7]. The balance between endogenous nucleoside levels and their synthetic analogs significantly impacts the efficacy and toxicity of many oncological and antiviral treatments (Journal of Clinical Oncology) [8]. Furthermore, the transport and metabolism of these substrates are often dysregulated in diseases such as cancer and mitochondrial DNA depletion syndromes (Nature Reviews Cancer) [9].

Other names

Endogenous nucleosidesDeoxyribonucleosidesNucleoside salvage substratesPyrimidine and purine nucleosides

Antimetabolite drugs act as structural analogs that compete with these endogenous substrates for enzyme active sites (e.g., Thymidylate synthase, DNA polymerase) or for incorporation into nucleic acids, thereby inhibiting DNA synthesis and cell division.