Anemia is a major complication in patients with CKD, starting in pre-dialysis stages and worsening with worsening renal function and as dialysis is initiated. It can result in significant morbidity and impact quality of life. Current treatments include erythropoiesis stimulating agents (ESAs) which were first approved in 1989 with Amgen’s Epogen, which is usually given three times a week iv or sc. Longer acting versions such as darbepoetin alpha (Aranesp) and methoxy polyethylene glycol-epoetin beta (Mircera) can be injected once every week to once every 4 weeks [Egrie JC 2003; Locatelli F 2007]. Although very effective in treating anemia, ESAs are not as effective in overcoming anemia associated with inflammation, resulting in EPO hypo-responsiveness in a significant number of patients. This can lead to the requirements for higher doses to maintain Hb within the current guidelines [see KDOQI and KDIGO], which in turn can increase adverse effects such as hypertension and the risk of thromboembolic events [Adamson JW 2009; Pfeffer MA 2003; Krapf R 2009].
The discovery and characterization of the principal cellular oxygen sensor, hypoxia-inducible factor (HIF) by Gregg Semenza [Wang GL 1995] led to the understanding of how oxygen tension regulates EPO production and a host of other cellular events [ke Q 2006; Haas VH 2013]. Further discovery of the regulation of HIF by prolyl and asparaginyl hydroxylases (PHD and FIH) [Bruick RK 2001; Masson N 2003] led to the development of PHD inhibitors that are poised to become major contenders in the treatment of anemia of CKD and cancer. This article summarizes the mechanisms by which PHD inhibition and HIF stabilization improves anemia and the development status of the several PHD inhibitors vying for Health Authority approvals in the near future.
Hypoxia-Inducible Factor (HIF) and its Regulation:
Under hypoxic conditions, prolyl hydroxylases (PHDs) are inactive, so HIF is released and binds to hypoxia response elements (HRE) on the effector gene such as EPO and increases transcription and synthesis of EPO, which in turn stimulates red cell production. Under normoxic conditions, PHD adds hydoxl groups on two proline residues of HIF-1α, which allows it to bind von Hipple Lindau (VHL) tumor suppressor and prime it for ubiquitination, followed by proteasomal degradation. Inhibition of PHD stabilizes HIF and prevents its degradation, therefore leading to stimulation of erythropoietin gene transcription and EPO synthesis, followed by increased red cell production by the bone marrow.
HIF molecule is composed of α and ß subunits. HIF-2α is the HIF isoform responsible for regulation of EPO in the kidney (Bernhardt WM 2010). HIF-2α is also the dominant isoform responsible for regulation of iron metabolism genes and also stimulates EPO production (see above). HIF-1α responds to tissue ischemia and hypoxia to increase angiogenic factors (Muchnik E 2011). HIF-1α and HIF-1ß isoforms have different specificities to different prolyl hydroxylases (PHDs), enzymes that are responsible for degradation of HIF. In addition to its role in EPO production, HIF increases intestinal iron absorption via upregulation of duodenal divalent metal transporter 1 (DMT-1), the major iron transporter. It also downregulates hepcidin expression. Hepcidin, a peptide hormone that sequesters iron and prevents its absorption and/or release into the circulation, also downregulates DMT-1, so its reduction can further increase intestinal iron transport and absorption [Gupta N 2017]. These effects further complement the effect of PHD inhibition on EPO production. Accordingly, PHD inhibitor treatment results in the suppression of hepcidin levels and increased iron absorption and availability, so intravenous iron is not needed. When EPO is administered, it is necessary to administer intravenous iron to maintain adequate iron levels and maintain effectiveness. A major cause of EPO unresponsiveness or EPO resistance is “functional iron deficiency”, which is due to unavailability of iron despite adequate iron stores. This appears to be due to high levels of hepcidin, an iron regulatory peptide that can sequester iron and make it unavailable for use in erythropoiesis [Bernhardt WM 2010; Maxwell PH 2016; Gupta N 2017]. Other potential indications for PHD inhibitors: preclinical studies suggest roles in cytoprotective and neuroprotective actions of PHD inhibitors [Reischl S 2014].
HIF stabilizers as novel therapies for anemia and other conditions:
Several new molecular entities belonging to a novel class of drugs called hypoxia-inducible factor prolyl hydroxylase (HIF-PHD) inhibitors are currently in Phase 2 to late-stage development by several pharmaceutical and biotech companies. They include roxadustat, being developed by Fibrogen in collaboration with AstraZeneca, vadadustat, being jointly developed by Akebia, Otsuka and Mitsubishi Tanabe, daprodustat, being developed by GlaxoSmithKline (GSK), and molidustat being developed by Bayer Healthcare. All four compounds are taken orally but differ slightly in their pharmacokinetic properties.
HIF-PHD inhibitors are attractive molecules for the treatment of anemia of CKD as well as anemia due to other chronic diseases such as cancer and inflammation. They have the advantage of oral administration and the data published or communicated as news releases show that these drugs can correct anemia relatively rapidly without an increase in blood pressure. Interestingly, vadadustat was effective not only in anemic patients with ND-CKD, but it also resulted in increased Hb in dialysis patients, including nephric and anephric subjects (Bernhardt WM 2010). This suggests that the kidney’s ability to produce EPO is suppressed but not destroyed by CKD, and extrarenal erythropoiesis (e.g. in the liver) could be activated. The increase in Hb after HIF-PHD inhibition is associated with a much lower rise in serum EPO levels, within the range observed in normal individuals, whereas exogenous EPO administration causes a marked increase in serum EPO levels, which may be a contributing factor to some of the adverse events observed when high EPO doses have been administered in an attempt to increase Hb levels toward normal [Levin A 2007; 4 Pfeffer MA 2009].
An additional therapeutic area for PHD inhibitors and one that has become very limited for other ESAs (exogenous EPOs) is anemia of cancer (chemotherapy-induced anemia) and anemia of chronic disease [Nagel S 2010]. The mechanism of action of PHD inhibitors is such that they can overcome the resistance or unresponsiveness to EPO due to the inability of iron to be mobilized from stores. This class of drugs increases intestinal iron absorption and improves iron mobilization through reduction in hepcidin. The overall effect is to bypass the functional iron deficiency seen in anemia of chronic disease. The exogenous EPO levels needed to overcome “EPO unresponsiveness” are in the 100,000 IU range, and have been associated with significant adverse events, including cardiovascular events and mortality, leading to strict label restrictions on their use in chemotherapy-induced anemia in cancer patients [Szczech LA 2008; Rizzo JD 2010].
Based on available data, HIF-PHD inhibitors are generally well-tolerated with adverse events primarily related to the GI tract, including nausea and diarrhea in approximately 4% of treated subjects compared to 2% in placebo [Pergola PE 2016]. Because of theoretical enhancement of tumor growth and promotion of angiogenesis with HIF 1α and 2α overexpression, increased vigilance and continuous monitoring of off-target effects is warranted [Muchnick E 2011]. However, based on the data available to date, increased VEGF levels have not been reported in studies of HIF-PHD inhibitors under development [Gupta N 2017].
Key words: Anemia; erythropoietin; hypoxia-inducible factor; prolyl hydroxylase; HIF stabilizer; hepcidin
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