There are several key glycolytic enzymes that can be targeted pharmacologically in cancer treatment. The first orally administered small molecule that was tested in the early 1980’s to inhibit glycolysis was lonidamine, as an inhibitor of mitochondria-bounded hexokinase [
137]. Lonidamine was later tested in a randomized clinical trial for glioblastoma in combination with radiotherapy, but failed to show therapeutic improvement compared to radiotherapy alone [
138]. Following the failure of lonidamine, a structural analogue of glucose, 2-deoxy-D-glucose, was tested. Metabolically, it inhibits glycolysis via competitive inhibition of hexokinase 2 that controls the first rate-limiting step of glycolysis [
139‐
141]. It was also reported to reduce the endoplasmic reticulum stress-related pathways which is significantly correlated to the radioresistance of GSCs [
142]. Furthermore, 2-deoxy-D-glucose is also able to modify the DNA repair pathways to optimize radiotherapy in HGG treatment [
143]. Treatment with 2-deoxy-D-glucose is cytostatic and radiosensitizes a range of cancer cells including HGGs [
144]. These findings led to a phase I/II trials testing 2-deoxy-D-glucose in combination with radiotherapy in patients with HGG, which demonstrated this combined treatment was well tolerated without any acute toxicity or late radiation damage to the normal brain tissue [
145]. This combination therapy also resulted in a moderate increase in median survival with a significantly improved quality of life [
146]. Another leading compound 3-bromopyruvate, a pyruvate analogue, is both an alkylating agent and an inhibitor of hexokinase 2. 3-bromopyruvate inhibits tumor growth in a dose-dependent manner in vivo [
147], but its off-target effect as well as the inability to penetrate the blood brain barrier impeded its further application in HGGs [
148]. Another key enzyme that can be pharmacologically targeted is pyruvate dehydrogenase kinase, which controls the rate and amount of pyruvate entering the tricarboxylic acid cycle by negatively regulating pyruvate dehydrogenase activity. Inhibition of pyruvate dehydrogenase kinase shifts the glucose metabolism and increases oxygen consumption of tumor cells, which in turn inducing higher level of ROS, thus improving the radiosensitivity of the tumor cells [
149]. Dichloroacetate (DCA), a small molecule inhibitor of pyruvate dehydrogenase kinase (PDK) with the potential for such metabolic modulation, has been shown to reverse the Warburg effect, thereby inhibiting both tumor cell growth and angiogenesis [
150,
151]. By combining with radiotherapy, dichloroacetate enhances the radiosensitivity of several types of cancer in vitro [
152‐
154] and in vivo including adult and pediatric HGGs [
155,
156]. Dichloroacetate has been used as an orphan drug for various acquired and congenital disorders of mitochondrial metabolism in both adult and pediatric patients for decades and has recently been demonstrated to be feasible and well-tolerated in patients with recurrent HGGs in a recent Phase I clinical trial [
157]. In addition, a recent study has tested the efficacy of DCA in a small cohort of HGG patients, suggesting metabolic modulation through pyruvate dehydrogenase kinase inhibition as a novel therapeutic strategy for the treatment of this devastating brain tumor [
158]. Interestingly, the human toxicity from chronic DCA exposure is generally limited to a reversible peripheral neuropathy that is now known to be influenced by age. The dose-limiting neuropathy in the glioblastoma study above occurred at a DCA dose that is known to cause no side effects in children. The identical dose known to cause neuropathy in adults with mitochondrial disease has been safely given to children with congenital mitochondrial diseases for many years [
159]. These findings and clinical results suggest that trials targeting the pediatric brain tumor population with DCA may be warranted, particularly given the safety and reduced toxicity of chronic administration of DCA in children. Moreover, melatonin, the secretory product of pineal gland, has also been recently reported to promote the synthesis of acetyl-CoA from pyruvate by inhibiting PDK in breast cancer models [
160]. The inhibitory effect of melatonin on PDK not only reverses the Warburg effect, reduces tumor mass, and improves the sensitivity of tumor cells to chemoradiotherapy [
160,
161], it also leads to a circadian rhythm of glucose metabolism in these cancer cells [
162,
163].