TKT and glioblastoma: Various purine and pyrimidine analogs are compatible with HSV-TK SGT such as ganciclovir (GCV), ACV, and brivudin (BVDU).18,19 BVDU is an efficient substrate of HSV-TK and a potent inducer of cell death,20 but exhibits poor BE.21 GCV is a better substrate for HSV-TK compared with ACV and exhibits a greater BE compared to either ACV or BVDU.21,22 Valganciclovir (valGCV), an oral analog of GCV, has recently been shown to be suitable for long-term treatment in a GBM xenograft model.23