Date of Award
Fall 2022
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Experimental Pathology
First Advisor
Bindra, Ranjit
Abstract
Glioblastoma multiforme (GBM) is a devastating disease afflicting over 20,000 new patients per year in the US with a 5-year survival rate of ~5%. The current standard of care treatment for GBMs consists of radiation therapy with concurrent and adjuvant chemotherapy with temozolomide (TMZ). TMZ is a prodrug that generates methyl diazonium in vivo, which in turn methylates DNA. O6-Guanine is the clinically significant site of DNA alkylation by TMZ. In healthy cells these lesions are reversed by O6-methylguanine methyltransferase (MGMT), a suicide DNA repair enzyme. However, loss of MGMT expression is common in many cancers such as grade 2/3 gliomas (50–70%), glioblastoma multiforme (GBM; 50%), colorectal (40%), and various lung cancers (25–35%). Stupp and co-workers established that loss of MGMT in GBM confers sensitivity to TMZ, and a combination of radiation therapy and TMZ increases a median overall survival by 8 months. Thus, TMZ in combination with radiotherapy has emerged as the standard-of-care in patients with MGMT– GBM.The mechanism of toxicity of TMZ is complex. O6-Methylguanine lesions themselves are relatively nontoxic (replication bypass efficiency is near 100%). However, O6-methylguanine is mispaired by thymine during replication. This mispairing triggers activation of the DNA mismatch repair (MMR) pathway, which resects the thymine residue. However, thymine is re-inserted by DNA polymerase, ultimately leading to cycles of thymine resection and insertion. These “futile cycles” lead to strand breaks and apoptosis. Thus, the toxicity of TMZ requires an intact DNA MMR pathway. Unfortunately, approximately half of all glioma patients and 15–20% of GBM patients develop acquired resistance to TMZ through mutations of MMR factors, most commonly MSH2 and MSH6. Effective treatment options for recurrent GBM are essentially non-existent as second line therapies, such as lomustine, do not possess a favorable therapeutic index and are limited by off-target toxicity. We report the design, synthesis, and evaluation of novel MGMT-dependent, MMR-independent therapeutics. Like TMZ, the novel alkylating agent, KL-50, provides a potentially clinically useful therapeutic index, but, significantly, retains activity in the setting of MMR mutations. KL-50 is active in multiple unrelated cancer cell lines with genetic and pharmacologic modeling of MGMT–/MMR– status. Structure–activity and mechanism of action studies reveal that KL-50’s activity is due to deposition of a 2-fluoroethyl adduct deposited at the O6G position of DNA. This lesion can be reversed by healthy cells that express MGMT but in the absence of MGMT this lesion converts to a highly toxic DNA interstrand cross-link. KL-50 demonstrated robust in vivo efficacy, systemic tolerability, and CNS penetrance in the treatment of both MGMT–/MMR+ and MGMT–/MMR– tumors. KL-50 is amenable to rapid derivatization of the imidazotetrazine scaffold to improve physiochemical properties, such as CNS penetration. These results position KL-50 as the first potential imidazotetrazine clinical candidate for the treatment of drug resistant gliomas.
Recommended Citation
Lin, Kingson, "Design, Synthesis, and Characterization of Novel MGMT-Dependent, MMR-Independent Agents for the Treatment of Glioblastoma Multiforme" (2022). Yale Graduate School of Arts and Sciences Dissertations. 715.
https://elischolar.library.yale.edu/gsas_dissertations/715
