HONG KONG — A new Japanese and U.S. collaborative study showing that inhibition of the oxygen sensor prolyl hydroxylase domain-containing protein 2 (PHD2) in the liver enhanced uptake of lactate for glucose production and ameliorated lactic acidosis, suggests that PHD2 may be a viable new therapeutic target for patients with life-threatening lactic acidosis.

When oxygen availability becomes limited, the body’s organs and cells activate the hypoxic response to generate energy, which releases a large amount of lactate into the circulation as a result of anaerobic glycolysis.

The resulting low pH caused by the accumulation of lactic acid in the blood and tissues can be a serious and frequently fatal complication in some critically ill patients, such as those with severe heart or lung ailments and infectious diseases.

“Lactic acidosis is a lethal and critical medical condition, especially when it is complicated with severe infection such as sepsis,” said lead researcher Yoji Andrew Minamishima, an assistant professor in the Department of Biochemistry at Keio University School of Medicine and group leader of the Suematsu Gas Biology Project at the Japan Science and Technology Agency in Tokyo.

Lactic acidosis can also be caused by various genetic conditions such as biotinidase, multiple carboxylase and other metabolic enzyme deficiencies, or be due to various other conditions, including hypoxia, ethanol toxicity and diabetic ketoacidosis. Moreover, significant mortality due to drug-induced lactic acidosis has been reported in diabetes patients taking metformin.

However, apart from the direct removal of lactate from the blood using hemodialysis, which is difficult to perform and has little evidence for benefit, there are limited available treatment options for patients with severe lactic acidosis.

While oxygen inhalation and blood transfusion can be beneficial, “oxygen inhalation might activate PHD2 in the liver that kills the hypoxic response in the liver,” cautioned Minamishima. “Moreover, blood transfusion might produce more lactate, because anaerobic glycolysis is the only way for the red blood cells to generate energy in the form of ATP. Taken together, there are no therapies to reduce the blood lactate level directly, apart from hemodialysis.”

The new study has now shown that activating the hepatic hypoxic response by inhibiting PHD2 in mice enhances lactate uptake for gluconeogenesis, also known as the Cori cycle, and ameliorates lactic acidosis, the researchers reported in the Aug. 31, 2015, early online edition of Proceedings of the National Academy of Sciences.

The researchers initially investigated PHD2 inhibition as a potential therapeutic target for lactic acidosis “because PHD2 is the most prominent of the three hypoxia-inducible factor (HIF) prolyl hydroxylases in vivo, meaning that if you want to turn on an HIF-dependent hypoxic response, PHD2 blockade alone is sufficient,” Minamishima told BioWorld Today.

On safety, he noted that chronic inactivation of PHD2 in the whole body causes fatal cardiomyopathy and erythrocytosis or polycythemia with increased numbers of erythrocytes. “However, PHD2-inhibition for short periods, such for as the treatment for lactic acidosis, would be safe.

“We found that PHD2-null mouse embryonic fibroblasts [MEFs] produced more lactate than wild-type MEFs, as expected, whereas systemic inactivation of PHD2 in these mice did not cause hyperlacticacidemia,” or dangerously high buildup of lactic acid in the blood, he said.

Minamishima explained that PHD2-null mice are embryonically lethal due to placental and heart deficiencies, “so we are working on conditional knockout [KO] mice.”

The finding that systemic inactivation of PHD2 did not cause hyperlacticacidemia is significant because “even though you inactivated PHD2 in whole body, the amount of lactate production from all of the cells due to activated anaerobic glycolysis by PHD2-blockade [would be] expected to be less than the amount of lactate clearance by the liver,” he said.

“This unexpected observation led us to hypothesize that the hypoxic response activated in the liver enhances the Cori cycle, a lactate-glucose carbon recycling system between muscle and liver, and thereby decreases circulating lactate,” said Minamishima.

Consistent with that hypothesis, the researchers found that blood lactate levels measured after a treadmill or lactate tolerance test were significantly lower in PHD2-liver-specific KO (PHD2-LKO) mice than in control mice.

An in vivo labeled lactate incorporation assay further revealed that the livers of PHD2-LKO mice produce significantly more glucose derived from labeled lactate than control mice, suggesting that blockade of PHD2 in the liver ameliorates lactic acidosis by activating gluconeogenesis from lactate.

Moreover, PHD2-LKO mice were resistant to lactic acidosis induced by the injection of a lethal dose of lactate, displaying a significant elongation of survival, while oral administration of a PHD inhibitor improved survival in a mouse model of endotoxic shock, which frequently triggers lactic acidosis.

In that experiment, the researchers used an investigational, orally active HIF-PHD inhibitor called GSK360A, which “significantly improved the 72-hour survival rate from 36.4 percent to 90 percent,” said Minamishima, noting that other PHD2 inhibitors are now in development or have entered clinical trials.

For example, U.S. biotech company Fibrogen Inc. and Japanese partner Astellas Pharma Inc. currently have an oral HIT-PHD2, FG-4592, in a phase III trial for the treatment of anemia in chronic kidney disease patients. (See BioWorld Today, Dec. 13, 2012.)

Minamishima is optimistic that PHD2 inhibitors might also improve survival in humans with lactic acidosis. “However, if the pharmaceutical companies judge that the market size for lactic acidosis is too small, they will not cooperate with us” in developing such an agent.

Nevertheless, “our findings suggest that PHD2 serves as a viable drug target for the treatment of life-threatening lactic acidosis, which is a frequent complication of severe infectious and ischemic diseases, as well as of [metformin] treatment in patients with diabetes with renal failure.”

Asked about his group’s future plans in that research area, Minamishima told BioWorld Today, “We have realized that energy management by hypoxic response is totally different between hepatocytes and that seen in other cell types. Therefore, we want to know about more details regarding the energy metabolism system of hepatocytes and hypoxic response.” //