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Assessing The Potential Of Gene Therapy In PAD And Wound Care: Is HGF The Answer?
According to the latest data, every 1.2 seconds worldwide someone develops a diabetic foot ulcer (DFU), every seven seconds someone dies from diabetes and every 20 seconds someone undergoes an amputation.1 In fact, one-third of the annual direct costs for diabetes in the United States are attributed to lower extremity care, and the five-year mortality rate following a major lower extremity amputation (LEA) is 56.6 percent, which is second only to lung cancer at 80 percent.2,3 Remarkably, lower extremity complications of diabetes now constitute a top ten global burden of disability according to the World Health Organization.4,5 Thus, treatment of the diabetic foot is common, complex and costly.
In the lower extremity, peripheral arterial disease (PAD) presents clinically as intermittent claudication, rest pain and tissue loss with or without gangrene, and occurs in up to 50 percent of patients with diabetes and a foot ulcer.6,7 The extent of ischemic symptoms in the lower extremity closely relate to the location of the vascular disease or lesion, as well as the presence and effectiveness of the surrounding collateral circulation. Intermittent claudication symptoms include cramping, pain or fatigue in the muscles of the leg that are present with ambulation, but relieved by rest.
Despite common misconceptions even today, lower extremity ischemia is the result of both atherosclerotic macrovascular disease and microcirculatory dysfunction, and patients with diabetes typically suffer from tibial and peroneal arterial disease.8,9 Although tibial and peroneal disease usually do not result in claudication, some patients do complain of foot pain or numbness while walking. However, nocturnal muscle cramping, common in patients with diabetes, should not be mistaken for intermittent claudication. While claudication can cause pain with ambulation, it rarely progresses to limb-threatening ischemia, and often responds to conservative measures such as exercise training and cessation of smoking.8,9 Furthermore, rest pain, which typically occurs in the distal foot and toes, occurs in patients with more severe vascular disease. Neuropathy from diabetes may sometimes mask symptoms of claudication or rest pain because of decreased sensation in these patients. The result of even more severe disease can lead to tissue loss, including foot ulceration or gangrene.8,9
The standard therapy for critical limb ischemia (CLI) is revascularization, either by endovascular technique or open bypass surgery, and there has historically been no effective medical management of the condition. In fact, revascularization options may be limited and associated with high rates of morbidity in older patients with significant comorbidities. For example, as demonstrated in the Bypass Versus Angioplasty in Severe Ischemia of the Leg (BASIL) trial, open surgical options are not available in up to 50 percent of patients with CLI due to concurrent medical comorbidities or lack of a suitable autogenous conduit or distal target vessel.10
The ultimate goal of revascularization is to restore adequate perfusion distal to an occlusion in order to relieve symptoms and promote healing. As mentioned previously, because patients with diabetes typically suffer from tibial and peroneal disease, the most effective bypass typically involves restoring blood flow to the dorsalis pedis or the posterior tibial artery at the level of the ankle using an autologous vein.11 In addition to open surgical reconstruction for effective revascularization of the diabetic foot, endovascular interventions have emerged as a useful tool for vascular surgeons and interventionists. In fact, balloon angioplasty and stenting have been shown to be well-suited for focal, short segment iliac stenoses or occlusions, which occur in 10 to 20 percent of patients with diabetes.12 And, while the morbidity associated with open bypass procedures can be quite significant, including high readmission rates, the need for re-operation and slow rates of healing,13 less invasive endovascular therapy can represent a preferred alternative. Yet, in our observation, the consensus relating to endovascular interventions for CLI remains mixed. And, based on results from the BASIL trial, investigators concluded that angioplasty should be the first intervention chosen for patients with significant comorbidities and with a life expectancy of less than one to two years.10 Additionally, they recommend bypass first for patients expected to live longer than two years.10
More recently, the term chronic limb threatening ischemia (CLTI), has emerged as a more appropriate term that more accurately reflects the varying degrees of ischemia in patients with diabetes that can delay or prevent wound healing. In addition, it accounts for the non-critical nature of limb ischemia, whereby revascularization is not urgently needed for limb preservation, and it addresses the fact that amputation is not based solely on the severity of ischemia, but also on the wound characteristics, including presence of infection (see photo on left above).14,15
What Does The Literature Reveal About Gene Therapy?
So, what about patients who do not meet the criteria for revascularization or are deemed too high-risk to undergo such procedures? Gene therapy may represent a viable alternative. Therapeutic angiogenesis is a type of therapy that attempts to improve perfusion to ischemic tissue through the formation of new blood vessels from preexisting ones.16 Furthermore, it can be mediated through the delivery of recombinant proteins, cell therapy or gene transfer.17–23 The use of gene therapy for patients with CLI shows substantial promise in terms of improving ulcer healing or localized blood flow. Hepatocyte growth factor (HGF), one such angiogenic protein, regulates multiple genes involved in angiogenic processes by activating the transcription factor Ets-1 through the c-Met receptor.24
Furthermore, HGF is a promoter and enhancer for the development of new blood vessels, and its expression appears to be downregulated in patients with CLI.25,26 In a double-blind, randomized, placebo-controlled multi-center trial (HGF-STAT trial) conducted by Powell and colleagues, investigators evaluated the safety of intramuscular injections of naked plasmid encoding human HGF gene (Beperminogene perplasmid, AMG0001) to improve limb perfusion in patients with CLI.24 Eligible patients were randomized to receive placebo or one of three dose regimens of HGF plasmid by intramuscular injection. Follow up then took place for 12 months, with subsequent measurements at weeks one, two, three, four, five and seven, and months three, six and 12. Assessment of the safety of the injections took place through multiple evaluations including: the number of adverse events; concomitant medications taken by the subject; electrocardiogram (ECG); blood chemistry; hematology; coagulation and urinalysis; vital signs; physical examination; cancer and retinopathy screening; and assays for the HGF plasmid, HGF protein and HGF antibodies. Researchers measured the effect of HGF plasmid on the limb perfusion by transcutaneous oxygen tension (TcPO2).24
The authors concluded that intramuscular injection of HGF plasmid was safe and well-tolerated by trial participants.24 In addition, of the 73 patients available for efficacy analysis, patients treated with high-dose HGF showed a significant increase in TcPO2 at six months as compared to the placebo group. As far as wound healing, researchers noted no significant differences between the groups in terms of the incidence of complete wound healing, major amputations or death at six months. Notably, patients with a wide range of ulcer types enrolled in the study, including gangrene (Rutherford category 6) and patients with combined CLI and venous stasis ulcers.24
The same group conducted a follow-up Phase II study in order to determine the optimal location and delivery technique for HGF plasmid, as well as to assess the effect of HGF gene therapy on safety, wound healing and toe brachial index (TBI) in patients with CLI.27 For this study, enrolled patients with Rutherford categories 5 and 6 received intramuscular injections of HGF plasmid every two weeks based on arteriographically defined vascular anatomy specific to the patient, via guidance from duplex ultrasound, and as determined by a committee of vascular experts. Thus, each individual patient’s vasculature determined the injection location. For example, for patients with only tibial occlusive disease, administration of the injections were below the knee, and for patients with combined superficial femoral artery and tibial disease placement was below and above the knee.27
The study endpoints included wound healing as defined by change in ulcer size at three and six months, decreased incidence of major amputation, improved rest pain as measured by the visual analogue scale (VAS), as well as ankle brachial index (ABI) and TBI at three and six months.27 Notably, results demonstrated a significant improvement in TBI from baseline to six months in the HGF-treated group compared with placebo as well as an improved VAS. Ulcer healing at 12 months occurred in 31 percent of the HGF group compared to zero percent in the placebo group, and there was no difference in major amputation of the treated limb (HGF 29 percent versus placebo 33 percent) or mortality at 12 months (HGF 19 percent versus placebo 17 percent) between the study groups. The authors concluded that HGF gene therapy using a patient vascular anatomy-specific delivery technique was safe, maintained limb perfusion and decreased rest pain in patients with CLI.27
A subsequent Phase I study conducted by Cui and colleagues assessed the efficacy of injecting intramuscularly a different naked plasmid DNA (pUDK-HGF) for expressing HGF in patients with CLI.28 Enrolled subjects in this trial received local intramuscular injections of pUDK-HGF in the calf and/or thigh muscles, randomized into four different dose groups. In contrast to the previously mentioned study by Powell and team,24 patients in this trial received local intramuscular injections in the calf and/ or thigh muscles on day one and day 15. Determination of the first injection site was based on the vascular anatomy, two cm above the start of the stenosis or occlusion, with subsequent injections distributed along the course of the blood vessel at multiple sites (eight to 32 sites), a minimum of two cm apart. The authors found that intramuscular injection of pUDK-HGF was safe and resulted in improved healing of five patients with gangrene, as well as increased values of ABI and TcPO2.28
Another multicenter, randomized, double-blind, placebo-controlled trial conducted by Shigematsu and colleagues evaluated the efficacy and safety of intramuscular injection of HGF plasmid (AMG0001) in patients with CLI who were not eligible for revascularization.29 As opposed to previous studies, the primary endpoint of this trial was improvement of rest pain in Rutherford 4 patients without ulcers, or the reduction of ulcer size in Rutherford 5 patients. The secondary endpoints included changes in ABI, incidence of amputation and quality of life (QOL). Results demonstrated an overall improvement rate of Rutherford 4 and 5 patients in the HGF group (70.4 percent) versus placebo (30.8 percent). In fact, HGF plasmid therapy achieved 100 percent improvement after 12 weeks in Rutherford 5 patients. Furthermore, the HGF plasmid also improved QOL, with no reported major safety issues.29
A study investigating the safety and efficacy of prolonged administration of the HGF plasmid AMG0001 in patients with ischemic rest pain and/or a lower extremity ulcer or gangrene showed an increase in TBI, ankle pressure, and toe pressure in the AMG0001 group compared to placebo.30 For the trial, eligible patients received AMG0001 or placebo every two weeks by intramuscular injection to the target limb during two cycles of treatment. The exact anatomical location for the injections was based on review of vascular anatomy as determined by a central committee. Although the study was terminated early due to low enrollment, prolonged exposure to AMG0001 intramuscular injection showed no increased risk of any delayed adverse events.30
What New Research Is In Progress On Gene Therapy And Wound Healing?
More recently, a Phase II double-blind, randomized, placebo-controlled study is underway with the aims of assessing the efficacy and safety of AMG0001 for gene therapy in order to improve ulcer healing and perfusion in subjects with lower extremity wounds (NCT04267640).31 Collategene® (AMG0001) is already approved in Japan for the treatment of ulcers in patients with chronic arterial occlusion who have demonstrated an inadequate response to standard pharmacotherapy and difficulty in undergoing revascularization.
The purpose of the study is to evaluate patients with mild to moderate CLTI, as defined by the WIfI classification, grade 1 or 2.31 Eligible subjects are those patients with mild-to-moderate ischemia and a lower extremity ulcer who likely do not warrant immediate revascularization or an amputation. This point is important, because revascularization is usually only considered in low-risk patients when there has been a failure of standard wound care to achieve healing. Thus, proactively treating these CLTI patients early in the disease process to improve limb perfusion truly represents an unmet need, whereby no such treatment for CLTI is available or currently approved for use in the United States.
Final Thoughts
Patients with mild-to-moderate ischemia, who present with a lower extremity ulcer, but do not warrant immediate revascularization or an amputation may benefit from medical therapy aimed at improving limb perfusion. Gene therapy in this population may represent a novel way to potentially avoid lower extremity amputation and increase ulcer-free days.
Dr. Armstrong is Professor of Surgery at the Keck School of Medicine at the University of Southern California. He is the Director of the Southwestern Academic Limb Salvage Alliance (SALSA).
Dr. Adams is a Surgery Fellow at the Professional Education and Research Institute in Roanoke, Va.
Dr. Driver is a Surgery Fellow at the Professional Education and Research Institute in Roanoke, Va.
Dr. Isaac is Co-Director of Research with Foot & Ankle Specialists of the Mid-Atlantic (FASMA).
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30. Powell R, Armstrong D, Conte M, Zelen C, et al. Safety and efficacy of prolonged administration of hepatocyte growth factor (HGF) plasmid (AMG0001) in patients with chronic limb threatening ischemia (CLTI). Presented at DFCon 2020. October 8-10, 2020. Los Angeles, Calif.
31. Study of AMG0001 to improve ulcer healing and perfusion in subjects with peripheral ischemic ulcers. Clinicaltrials.gov identifier: NCT04267640. Available at: https://clinicaltrials.gov/ct2/show/NCT04267640. Updated March 22, 2021. Accessed July 8, 2021.
