Temozolomide

Temozolomide is an oral activity alkylating agent that induces the formation of O6-methylguanine in DNA, which mispairs with thymine during the following cycle of DNA replication, leading to activation of the apoptotic pathways, Temozolomide could crosses the blood-brain barrier and is indicated for malignant gliomas and metastatic melanomas ^[1]. Temozolomide induced cell cycle arrest at G2/M and to eventually lead to apoptosis ^[2]. At physiologic pH it is converted to the short-lived active compound, MTIC. MTIC is further hydrolyzed to 5-amino-imidazole-4-carboxamide (AIC) and to methylhydrazine. The cytotoxicity of Temozolomide is mediated by its addition of methyl groups at N7 and O6 sites on guanines and the O3 site on adenines in genomic DNA. Alkylation of the O6 site on guanine leads to the insertion of a thymine instead of a cytosine opposite the methylguanine during subsequent DNA replication, and this can result in cell death ^[3].

Lymphoma cell counts performed 72 hours after treatment showed that the IC50 of Temozolomide was 44 µM (35-58 µM) when used alone and 16 µM (12-26 µM) when combined with NU1025 ^[1]. A time-related response in DNA strand-break formation was observed in the U251MG glioblastoma cells treated with Temozolomide (100 µM) alone ^[4]. Temozolomide was particularly evident when U-118 cells were incubated with Temozolomide concentrations of >100 µM. When U-118 cells were incubated with Temozolomide (250 µM) the proliferation rate was inhibited by 43.2% ^[5].

Temozolomide consistently demonstrates reproducible linear pharmacokinetics with approximately 100% p.o. bioavailability ^[6]. In the early stage s.c. implanted SNB-75 astrocytoma model, a 400-mg/kg dose of Temozolomide administered on Day 5 produced 10 of 10 Day 54 tumor-free mice. In later staged s.c. U251 and SF-295 glioblastoma models, a single 600-mg/kg dose produced 9 of 10 Day 86 and 2 of 10 Day 40 tumor-free mice, respectively. In the latter group, a tumor growth delay of >315% was attained ^[7]. Treatment of CEP 6800 (30 mg/kg) with Temozolomide (17 and 34 mg/kg) resulted in 100% complete regression of U251MG tumors by day 28 versus 60% complete regression caused by Temozolomide alone ^[4].

References:
[1]. Tentori L, Leonetti C, Scarsella M, et al. Combined treatment with temozolomide and poly (ADP-ribose) polymerase inhibitor enhances survival of mice bearing hematologic malignancy at the central nervous system site[J]. Blood, The Journal of the American Society of Hematology, 2002, 99(6): 2241-2244.
[2]. Baer J C, Freeman A A, Newlands E S, et al. Depletion of O6-alkylguanine-DNA alkyltransferase correlates with potentiation of temozolomide and CCNU toxicity in human tumour cells[J]. British journal of cancer, 1993, 67(6): 1299-1302.
[3]. Lee S Y. Temozolomide resistance in glioblastoma multiforme[J]. Genes & diseases, 2016, 3(3): 198-210.
[4]. Miknyoczki S J, Jones-Bolin S, Pritchard S, et al. Chemopotentiation of temozolomide, irinotecan, and cisplatin activity by CEP-6800, a poly (ADP-ribose) polymerase inhibitor[J]. Molecular cancer therapeutics, 2003, 2(4): 371-382.
[5]. Carmo A, Carvalheiro H, Crespo I, et al. Effect of temozolomide on the U-118 glioma cell line[J]. Oncology letters, 2011, 2(6): 1165-1170.
[6]. Friedman H S, Kerby T, Calvert H. Temozolomide and treatment of malignant glioma[J]. Clinical cancer research, 2000, 6(7): 2585-2597.
[7]. Plowman J, Waud W R, Koutsoukos A D, et al. Preclinical antitumor activity of temozolomide in mice: efficacy against human brain tumor xenografts and synergism with 1, 3-bis (2-chloroethyl)-1-nitrosourea[J]. Cancer research, 1994, 54(14): 3793-3799.

替莫唑胺是一种口服活性烷化剂,可诱导 DNA 中 O6-甲基鸟嘌呤的形成,其在随后的 DNA 复制周期中与胸腺嘧啶错配,从而激活细胞凋亡途径,替莫唑胺可穿过血脑屏障并适用于恶性神经胶质瘤和转移性黑色素瘤^[1]。替莫唑胺诱导细胞周期停滞在 G2/M 期并最终导致细胞凋亡^[2]。在生理 pH 值下,它会转化为短寿命的活性化合物 MTIC。 MTIC 进一步水解为 5-氨基-咪唑-4-甲酰胺 (AIC) 和甲基肼。替莫唑胺的细胞毒性是通过在基因组 DNA 中鸟嘌呤的 N7 和 O6 位点以及腺嘌呤的 O3 位点添加甲基来介导的。鸟嘌呤上 O6 位点的烷基化导致在随后的 DNA 复制过程中插入胸腺嘧啶而不是甲基鸟嘌呤对面的胞嘧啶,这可能导致细胞死亡^[3]。

治疗后 72 小时进行的淋巴瘤细胞计数显示,替莫唑胺的 IC50 单独使用时为 44 µM (35-58 µM) 和 16 µM (12-26 µM) 与 NU1025 结合时 ^[1]。在单独使用替莫唑胺 (100 µM) 处理的 U251MG 胶质母细胞瘤细胞中观察到 DNA 链断裂形成的时间相关反应^[4]。当 U-118 细胞与浓度 >100 µM 的替莫唑胺一起孵育时,替莫唑胺特别明显。当 U-118 细胞与替莫唑胺 (250 µM) 一起孵育时,增殖率被抑制了 43.2% ^[5]。

替莫唑胺始终显示出可重复的线性药代动力学,约为 100 % 口服生物利用度^[6]。在早期阶段 s.c.在植入 SNB-75 星形细胞瘤模型后,在第 5 天施用 400 mg/kg 剂量的替莫唑胺产生了 10 只第 54 天无肿瘤小鼠中的 10 只。在后来的 s.c. U251 和 SF-295 胶质母细胞瘤模型,单次 600 mg/kg 剂量分别产生 10 天 86 中的 9 只和 10 天 40 中的 2 只无肿瘤小鼠。在后一组中,肿瘤生长延迟达到 >315%^[7]。用替莫唑胺（17 和 34 mg/kg）治疗 CEP 6800（30 mg/kg）导致 U251MG 肿瘤在第 28 天时 100% 完全消退,而单独使用替莫唑胺导致 60% 完全消退^[4].