Thrombin Peptide Chrysalin Stimulates Healing of Diabetic Foot Ulcers in a Placebo-Controlled Phase I/II Study

ABSTRACT

Thrombin and thrombin peptides play a role in initiating tissue repair. The potential safety and efficacy of TP508 (Chrysalins) treatment of diabetic foot ulcers was evaluated in a 60-subject, prospective, randomized, double-blind, placebocontrolled phase I/II clinical trial. Chrysalins in saline or saline alone was applied topically, twice weekly, to diabetic ulcers with standardized care and offloading. A dose-dependent effect was seen in the per-protocol population where 1 and 10 mg Chrysalins treatment resulted in 45 and 72% more subjects with complete healing than placebo treatment. Chrysalins treatment of foot ulcers more than doubled the incidence of complete healing (p < 0.05), increased mean closure rate
80% (p < 0.05), and decreased the median time to 100% closure by
40% (p < 0.05). Chrysalins treatment of heel ulcers within this population resulted in mean closure rates 165% higher than placebos (p < 0.02) and complete healing in 86% (6/7) of ulcers compared with 0% (0/5) of placebo ulcers (p < 0.03). Local wound reactions and adverse events (AEs) were equal between groups with no reported drug-related changes in laboratory tests or serious AEs. These results indicate the potential safety and efficacy of Chrysalins for treatment of diabetic foot ulcers.

Chronic diabetic ulcers of the lower extremities represent a major healthcare problem today, with over 850,000 diagnoses made in the United States each year.1,2 Because of the increasing incidence of diabetes, the magnitude of the challenge presented to the healthcare system by chronic foot and leg ulcers is also expected to increase.1,2 Chronic diabetic ulcers not only negatively impact the quality of life but can also lead to amputation and an increased likelihood of death.

Healing a chronic diabetic ulcer is an expensive, time consuming, and complicated task. A significant fraction of these ulcers do not heal or do not remain healed, and become chronic wounds that endure for months or years. The cost to the healthcare system is estimated at greater than 10 billion dollars per year, in addition to untold losses in workplace productivity.3,4 Diabetic ulcers are distinguished from acute wounds in healthy individuals by several factors that stem from the underlying pathology of diabetes including the aging of tissues, hypoxia, and infection. In addition, diabetics exhibit various degrees of peripheral neuropathy. Diabetics also have a dysfunctional endothelium, which fails to respond to various growth factors and angiogenic stimuli,8,9 and increased levels of metalloproteinases that degrade matrix molecules and decrease the half-life of growth factors in wound fluid. The impaired wound environment characteristic of chronic, nonhealing ulcers has led to development of a standard regimen of chronic wound care that includes appropriate wound bed preparation, moist wound coverings, and pressure off-loading. For many larger ulcers, treatment in wound-care centers now often includes use of bioactive skin substitutes, delivery of autologous platelet concentrates, or application of a therapeutic agent such as becaplermin (Regranexs OMJ Pharmaceutical Inc., San Germa' n, Puerto Rico). Although these therapeutic options have shown potential efficacy in clinical trials and received FDA approval, they do not appear to have been widely accepted as standard of care.

The reasons why therapeutic alternatives approved by the FDA for diabetic foot ulcers have not been widely accepted include limited benefit over standard of care, treatment regimens that require extensive debridement, daily treatment, and/or repeated visits to specialized wound care facilities. Clinical trials with becaplermin (Regranexs), using a regimen that included daily treatment for 12 hours, rinsing with saline, and rebandaging, e.g., showed complete healing in
50% of subjects relative to 35% who were treated with good clinical practice and placebo gel. A subsequent post hoc analysis suggested that the efficacy of becaplermin was seen in subjects whose wounds were debrided almost every week. In both the home health setting and in many wound clinics, treatment schedules and debridement fall short of the regimen that may be required for becaplermin efficacy. Therefore, a more ideal therapeutic for diabetic foot ulcers may be one that can be easily applied in the home care environment, can be applied less often without reducing efficacy, and is less dependent upon surgical debridement for its efficacy.

Chrysalins (TP508) is a 23-amino acid peptide representing the natural sequence of amino acids of human thrombin identified as the thrombin-binding domain for a specific class of thrombin receptors on fibroblasts and other cells.16 Early studies showed that thrombin, the serine protease responsible for fibrin clot formation, initiated cell proliferation and other cellular postclotting events through a growth factor-like mechanism that involved its binding and activation of specific thrombin receptors on the surface of fibroblasts and other cells. Although many of the cellular effects of thrombin appear to require proteolytic activity and activation of proteolytically activated receptors,20 studies show that binding of thrombin or thrombin derivatives without proteolytic activity promotes a number of cellular events involved in tissue repair and wound healing. These observations have led to the hypothesis that nonproteolytic peptide fragments of thrombin released from a fibrin clot during early stages of wound repair may modulate inflammation and promote healing.

Unlike thrombin, which is activated at the site of injury, the Chrysalins peptide has no enzymatic activity and does not promote or interfere with blood coagulation.16 Preclinical safety studies have shown that the peptide can be injected intravenously or intraperitonealy at doses of up to 25 mg/kg with no adverse effects, that it is classified as a nonsensitizer based on hamster skin sensitivity testing, and that topical treatment of open porcine wounds (followed upon wound closure with dermal injection at the wound site) of 100 mg/day for 20 weeks had no apparent negative effects (OrthoLogic Corp., unpublished results).

In full-thickness incisional wounds in rats, a single topical application of Chrysalins increased the breaking strength of wounds by approximately 80% over saline controls when measured at day-7 postincision.25 Relative to control breaking strength, this single application of Chrysalins shifted the healing curve forward by approximately 4 days. Significant effects of Chrysalins were also seen on incisional wounds in rats with radiation-induced healing impairment. In larger full-thickness excisional animal wounds in normal and ischemic skin, a single topical application of Chrysalins also accelerated wound closure. In all of these model studies, Chrysalins accelerated recruitment of inflammatory cells to the wound site, shortened the inflammatory phase, and promoted early revascularization of the tissues. Chrysalins also accelerated repair of rat fresh fractures and promoted bone formation in rabbit critical size segmental bone defects and in a rabbit model of distraction osteogenesis. Thus, this molecule may serve as a natural initiator of tissue repair in a number of tissues.

Based on preclinical studies, we hypothesized that Chrysalins may improve repair quality and accelerate the rate of tissue repair following surgical or traumatic acute tissue damage and reinitiate healing of chronic ulcers where normal repair processes are disrupted. We now report the results of the first human phase I/II pilot clinical trial designed to evaluate the safety and efficacy of Chrysalin s in the treatment of chronic diabetic lower extremity and foot ulcers. Modeled after other diabetic foot ulcer clinical trials, but with expanded entry criterion including inclusion of larger, more severe (Grade III) ulcers, ulcers on the leg and ankle, and those with O2 tension (TcPO2) levels of between 20 and 30 mmHg, this trial was designed to determine whether twice-weekly application of Chrysalins could cause healing of diabetic ulcers and help us better define the role of thrombin peptides in tissue repair.

MATERIALS AND METHODS

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Dr. Jeffrey Stone, DO, MPH, FCCWS, specializing in Wound Care and Hyperbaric Medicine, serves as Medical and Fellowship Director for the Hyperbaric Medicine Unit at the Institute for Exercise & Environmental Medicine at Texas Health Dallas. Dr. Stone received his medical degree from Western States University and received a Master of Public Health degree from Harvard University. Dr. Stone completed a residency in aerospace medicine at Brooks Air Force Base and a fellowship in hyperbaric medicine and Wound care at the U.S. Air Force School of Aerospace Medicine. He is one of the first 60 physicians in the U.S. to receive his certification in Wound Care. Dr. Stone currently provides care to patients in acute care, LTAC, long term care facilities SNF and Nursing Home) and outpatient care.

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