Heart failure (HF) is the most frequent cardiovascular diagnosis and exacts significant health and financial costs around the globe. It is estimated that at least 26 million people worldwide are living with HF, including nearly 6 million in the United States.1, 2 One in nine U.S. deaths in 2009 included heart failure as a contributing cause and about 50 percent of people in the U.S. with HF die within five years of diagnosis.2 The annual cost of HF-related healthcare services, medication and missed days of work is estimated at $40 billion in the United States and $108 billion globally.3, 4 Quality of life in HF patients is frequently worse than many other chronic diseases and comorbidities are common.5-7 The challenges of HF are expected to grow, as it is estimated that more than 8 million people in the United States alone will have HF by 2030.2 Current therapies improve quality of life in the short-term and have improved long-term survival but a significant number of patients have Class 3 HF despite optimal medical and device therapy. These patients have limited treatment options beyond heart transplant and left ventricular assist devices (LVAD). New therapeutic approaches that address the underlying causes of HF are needed to improve patient outcomes.


Complex biology provides multiple targets for HF therapy

Heart failure is a complex disease process and multiple pathways contribute to its development and progression. Myocardial ischemia is frequently an issue in both ischemic and non-ischemic cardiomyopathy as well as HF with preserved and/or reduced ejection fraction. Myocardial ischemia results in insufficient oxygen and nutrients and leads to hypoxia, cardiomyocyte and fibrosis, which all contribute to the progression of heart failure. More effective angiogenesis may prevent this progression. Cell homing also plays a critical role, as injured cardiac tissue secretes factors that lead to the recruitment, proliferation, migration and differentiation of progenitor cells that can help repair tissue damage. Stromal cell-derived factor (SDF)-1 has been shown to play an important role in cardiac repair by mediating cell homing.10 Mitochondrial energy generation is also impaired in HF, leading to decreased contractility and adverse changes to cardiac architecture.11 Scar tissue formed in response to cardiomyocyte injury or death can compromise the heart’s mechanical strength or electrical signaling results in myocardial infarction. Inflammatory responses to cardiac tissue damage can promote inappropriate and chronic inflammation and the expression of pro-inflammatory molecules that lead to pathologic changes to cardiac architecture.12, 13

These pathways offer a variety of potential new targets for therapeutic intervention to prevent the development and progression of HF. This opens the door to the development of novel therapies that address the underlying molecular and cellular causes of disease rather than treating HF symptoms alone.


HF gene therapy

After decades of development, gene-based therapies are now validated therapeutic modalities for the treatment of inherited retinal disorders and cancer and are undergoing clinical evaluation in a variety of inherited, acute and chronic diseases. Nearly two dozen single gene-based therapies for HF have been evaluated in clinical trials.14 Genes evaluated as monogenic gene therapy for HF in clinical trials include vascular endothelial growth factor (VEGF) and fibroblast growth factor type 4 (FGF4) to promote angiogenesis; adenylyl cyclase type 6 (AC6) and sarco/endoplasmic reticulum Ca2+-ATPase type 2 (SERCA2) to improve cardiac calcium homeostasis, which plays a critical role in the contraction and relaxation of heart muscle; and stromal cell-derived factor-1 (SDF-1) to improve cell homing and promote cardiac tissue repair. Late-stage trials of single gene therapies have yielded conflicting results, raising the question as to whether positively impacting a single pathway can be sufficient to overcome detrimental activity of other pathways that contribute to the development and progression of HF. Other potential limitations to HF therapies evaluated in clinical trials to date include the method of delivery, dose and the potency of vectors and gene products.


A multi-gene approach to a multimodal disease

Given the multiple molecular and cellular pathways active in HF, a multi-gene approach to HF gene therapy may be needed. Simultaneously delivering multiple genes that target diverse HF-related pathways has the potential to improve cardiac biology and function. A triple gene therapy approach (INXN-4001, Triple-Gene LLC, a majority-owned subsidiary of Intrexon Corporation) is currently in clinical development, with each of the genes targeting a specific HF-related pathway. The investigational drug candidate INXN‑4001 vector expresses: the S100A1 gene product, which regulates calcium-controlled networks and modulates contractility, excitability, maintenance of cellular metabolism and survival; SDF-1a which recruits stem cells, inhibits apoptosis and supports new blood vessel formation; and VEGF-165 which initiates new vessel formation, endothelial cell migration/activation, stem cell recruitment and tissue regeneration. The hypothesis is that the simultaneous delivery of multiple genes in a single vector would more effectively improve multiple aspects of cardiac function compared with single gene therapy. It is delivered by retrograde coronary sinus infusion of a triple effector plasmid designed with a self-cleaving linker to constitutively express human S100A1, SDF-1a and VEGF 165. This route is designed to allow for delivery of a dose to the ventricle which may help achieve improved therapeutic effect.

Several preclinical studies have set the foundation on which to advance a triple gene therapy for HF into the clinic.15-17 Using in vitro studies, transfecting cells derived from patients with dilated cardiomyopathy with a triple gene combination demonstrated improvement in contraction rate and duration, to the levels demonstrated by the control cells and did not result in increased cell death compared to controls.15 Studies in an Adriamycin-induced cardiomyopathy rodent model demonstrated triple gene therapy increased fractional shortening and myocardial wall thickness compared to controls.16 In addition, retrograde coronary sinus infusion of INXN-4001 in a porcine model of ischemic HF resulted in a cardiac-specific biodistribution profile.17

A Phase 1 clinical study has been initiated to evaluate the safety of a single dose of triple gene therapy in stable patients implanted with a LVAD for mechanical support of end-stage HF. An independent Data and Safety Monitoring Board agreed to proceeding to the second cohort following review of the data from the first cohort in the multi-site study.18 The study is ongoing and final results will help to inform our understanding of the potential that multi-gene therapy may play in the treatment of HF.


The future of gene-based therapies for HF

The recent FDA approvals of gene therapies for an inherited retinal disease and cancer are evidence that gene therapy is a valid therapeutic strategy. Realizing the potential of gene therapy in HF will require appropriately designed clinical trials, but several interesting approaches currently in development may prove to be effective. The results of the initial investigational drug INXN-4001 Phase 1 trial should provide insight into the safety of combining S100A1, SDF-1a and VEGF-165. Evaluation of additional multi-gene combinations will also be important for understanding which targeted pathways yield the greatest effects with respect to relevant clinical endpoints. Continued refinement and optimization of vector design and delivery methods will also be important for advancing further HF gene therapies from bench to bedside.