Erythropoiesis is a natural response to hypoxia
One of the body's natural HIF-mediated responses to hypoxia is increased erythropoiesis, the process whereby new oxygen-carrying red blood cells are formed in the bone marrow. Exposure to high altitude, for example, regularly leads to increased red blood cell mass, as shown by several studies of mountain climbing expeditions. Athletes try to capitalize on this phenomenon by training at high altitudes before major sports events to enhance their performance. More recently, this practice has been augmented by resting or sleeping in an atmosphere that mimics high altitude (e.g., hypobaric tents), but training at normal altitude. Athletes have maintained this practice for years, without apparent ill effects. Studies in human volunteers have shown that exposure to hypoxia three times a week for ninety minutes over a period of three weeks results in a marked elevation of red blood cell mass and hemoglobin (oxygen-carrying molecule located in red blood cells).

HIF-mediated erythropoiesis for the treatment of anemia
Anemia is a condition in which the blood cannot deliver enough oxygen to the cells and tissues, resulting in localized hypoxia. When blood delivered to the kidney does not contain enough oxygen, HIF activates the gene for erythropoietin (EPO), which stimulates red blood cell production, as well as genes involved in iron metabolism and hemoglobin synthesis, processes essential to proper red blood cell formation and maturation. By coordinating the entire erythropoietic process, HIF promotes the production of properly formed mature red blood cells, helping to restore delivery of sufficient oxygen to the body.

There are several lines of evidence supporting the tractability of a therapeutic approach to anemia based on the ability to selectively induce erythropoiesis through stabilization of HIF:

• Hypoxia studies in chronic kidney disease (CKD) - Numerous reports document the capacity of fibrotic kidneys in CKD patients undergoing dialysis or kidney transplantation to induce HIF stabilization, produce endogenous EPO, and activate HIF-dependent erythropoiesis in response to stimulation at high altitude or with hypoxia. Specifically, dialysis patients that were phlebotomized and exposed to systemic hypoxia experienced a 52-fold and 25-fold increase in circulating EPO, respectively; CKD patients after exposure to acute hypoxic stress experienced a 10-fold increase in circulating EPO; and dialysis patients exposed to high altitude experienced enhanced erythropoiesis. Use of hypobaric chambers, which simulate the effects of high altitude, has also been shown to correct hemoglobin deficiency in anemic CKD patients. In these patients, the liver may also be a significant source of endogenous EPO in addition to kidney, as the liver provides at least 10% of the total circulating EPO in humans.

• Therapeutic benefits of HIF stabilization validated - The mechanism of action two therapeutic agents, cobalt chloride and desferrioxamine, was recently discovered to act through the HIF pathway. In studies of dialysis and arthritis patients dating from as early as 1975, these drugs were shown to produce therapeutically relevant increases in both EPO and hematocrit (packed red blood cell volume). While various toxicities unrelated to their HIF-based mechanism of action have limited the use of these agents in medical settings, no adverse side effects have been found to be attributable to HIF stabilization.

• Chronic stabilization of HIF tolerated - In some medical conditions, HIF is directly involved in causing polycythemia (elevation of red blood cell mass above the upper limit of the normal range). In patients with Chuvash syndrome, for example, a congenital genetic mutation leads to a persistent physiological state from birth in which HIF is continuously active, albeit at low levels (normally, HIF is only active when the body experiences hypoxia). People with Chuvash syndrome lead essentially normal lives but exhibit polycythemia caused by a HIF-dependent constitutively higher production of endogenous EPO. The effects of sustained HIF stabilization can also be observed in patients with underlying chronic obstructive pulmonary disease (COPD), who experience airway obstruction and damage to the blood vessels in the lungs. This causes tissue hypoxia from lack of oxygen delivery. As the body tries to compensate for hypoxia, increased erythropoiesis occurs, resulting in secondary polycythemia in some COPD patients.

Selective erythropoietic HIF-stabilizing agents
FibroGen has identified a series of potent, orally bioavailable compounds that selectively activate HIF-mediated erythropoiesis, resulting in not only upregulation of endogenous EPO, but also in activation of enzymes involved in iron processing, transport, and utilization, activation of enzymes involved in heme conversion for synthesis of hemoglobin, and inhibition of the suppressive effects of inflammatory cytokines on erythropoiesis. FibroGen is developing certain erythropoietic HIF stabilizers in parallel for the treatment of anemia including FG-2216, the Company's lead compound in clinical development, and FG-4592 for the treatment of anemia due to iron processing deficiency.

Stabilization of HIF in a therapeutic setting is analogous to stabilization of HIF in Chuvash patients, in that both occur under normal oxygen concentrations. Studies of Chuvash patients have validated the role of HIF in erythropoiesis and the general principle of the safety of extended HIF stabilization. Although patients experience polycythemia from HIF stabilization, these patients do not experience increased angiogenesis or tumorigenesis, demonstrating the safety of long-term HIF stabilization. Similar to what is observed in Chuvash polycythemia patients, FG-2216 selectively stimulates erythropoiesis.