Practice question

Targeted killing arms tumor cells with enzymes converting systemically administered non-toxic prodrugs into potent cytotoxins achieving spatial specificity. Classic systems include HSV thymidine kinase type 1 phosphorylating ganciclovir 1000-fold higher affinity than mammalian kinase, E. coli cytosine deaminase codA converting 5-fluorocytosine to 5-fluorouracil inhibiting thymidylate synthase, bacterial nitroreductase NfsB reducing CB1954 to bifunctional alkylating crosslinking DNA. Expression restricted malignant cells using tumor-specific promoters survivin active G2/M, hTERT active 85 percent cancers silent differentiated tissue, CEA for colorectal carcinoma, or hypoxia-responsive elements containing HIF1 binding sites. Vector generally serotype 5 adenovirus injected intratumorally transduces fraction but toxic metabolites diffuse gap junction channels connexin 43 hexamers producing bystander effect killing neighboring nontransduced cells up to 10 diameters amplifying efficacy beyond transduction efficiency. Dividing cells preferentially incorporate nucleotide analogs prompting ATR-CHK1 DNA damage response p53 dependent apoptosis while quiescent normal tissue largely spared synergizing radiotherapy and checkpoint blockade. This mechanistic insight guides vector optimization, dosing strategies, and clinical safety monitoring essential for translational development and regulatory evaluation.