Diabetic peripheral neuropathy (DPN) afflicts up to 66% and 59% of patients with type 1 and type 2 diabetes respectively10. The peripheral neurons supplying the feet are the longest cells in the body. As such, they require adequate vascularization, mitochondrial support, and glucose and lipid metabolism, all of which are unfortunately disturbed in diabetes11. As pointed out by experimental and clinical and studies, the most important factor in DPN pathogenesis is hyperglycemia-mediated cellular-injury12, 13. Other factors include obesity, dyslipidemia, impaired neurotrophic support, and microangiopathy which induce oxidative stress, mitochondrial dysfunction, and inflammation11.

One way that hyperglycemia causes DPN is through the polyol pathway, wherein excessive intracellular neuron glucose is converted into sorbitol, and into fructose afterwards by aldose reductase and sorbitol dehydrogenase, respectively. The ultimate increase in polyol flux causes intracellular hyperosmolarity and compensatory efflux of osmolytes, one of which is myoinositol, resulting in phosphatidylinositol depletion, hindering the creation of adenosine triphosphate. All of these processes result in Na/K ATPase and protein kinase C dysfunction, impaired axonal transport, and neuronal breakdown. Furthermore, the reduction of glucose to sorbitol is also associated with NADPH consumption (which is required for the glutathione regeneration), thus contributing to oxidative stress14, 15.

In the presence of excess glucose, shunting to the hexosamine pathway also occurs. The conversion of fructose-6-phosphate to glucosamine-6-phosphate by glutamine: fructose-6-phosphate amidotransferase increases the production of uridine diphosphate N-acetylglucosamine that in turns modifies the transcription factor Sp1. This change results in altered gene expression and protein function. Consequently, vascular dysfunction, inflammation, and oxidative occurs11, 16.

Other than that, in a hyperglycemic environment, proteins, lipids, and nucleic acids can undergo irreversible non-enzymatic reactions which form advanced glycation products. These group of molecules, which are generated via the attachment of reactive carbohydrate groups, tend to affect cellular function negatively by disrupting the biological tasks of proteins. They also bind to their receptors which activates the intracellular signaling pathway, leading to inflammation, oxidative, stress, and nuclear degradation. This chain of events results in vascular dysfunction and deficits in neuronal conduction11, 14.

Peripheral vascular disease (PVD) is an important risk factor in the development of DFU. PVD is responsible for about 50% of DFU cases and 70% of deaths in type 2 diabetes2. Hyperglycemia, dyslipidemia, and insulin resistance found in diabetics foster the development of PVD through mechanisms similar to those found in coronary artery disease17. The risk of PVD is higher and earlier in patients with diabetes due to endothelial dysfunction, vascular smooth muscle dysfunction, inflammation, and hypercoagulability. Other than diabetes, high blood pressure, smoking, dyslipidemia, and older age are also risk factors for PVD18. Diabetics, who are at a higher risk of atherosclerosis, may suffer from acute or chronic ischemia which may also lead to gangrene. Poor peripheral vascularization also leads to a poor wound healing process and further worsens the situation. Once it has become a neuroischemic ulcer, the wound is at risk of infection. While ischemia is not an independent risk factor for amputation, simultaneous occurrence with infection greatly increases the risk of limb amputation. Deep infections, such as osteomyelitis or soft tissue infection, are an immediate cause of amputation in 25 — 50% of DFU cases19.

Other risk factors identified by studies include Charcot joints, a long duration of diabetes, retinopathy, nephropathy, hypertension, overweight, obesity, low socioeconomic level, foot deformity, and a history of previous DFUs and/or amputation20, 21, 22.

In DFUs, the lack of oxygen and nutrients hinders the epithelial cells from expressing growth factors, such as vascular endothelial growth factor (VEGF) and PDGF, which are vital for wound healing33, 34. With over 1500 bioactive factors found in platelets, aaPRP contains the much-needed growth factors which are beneficial for DFUs as the wounds are deficient in growth factors. Some examples of the growth factors include PDGF which has the ability to enhance collagen synthesis, fibroblast chemotaxis, and proliferation; VEGF which stimulates angiogenesis, the mitosis of endothelial cells, and increases vascular permeability; and the insulin-like growth factor (IGF) which promotes cell growth and differentiation, and also works with PDGF to stimulate collagen synthesis. Other growth factors such as transforming growth factor-β (TGF-β), epidermal growth factor (EGF), and fibroblast growth factor (FGF) are also present35. A summary of the regenerative functions of some of the bioactive molecules found in aaPRP are summarized in Table 1.

Other than growth factors, a diverse array of cytokines are also present in platelets. These proteins can help in the modulation of inflammation and regeneration. Some of these include interleukin (IL)-1 receptor antagonist, IL-10, tumor necrosis factor-α, IL-12p70, and IL-844, 45. For example, IL-10 is an anti-inflammatory cytokine that is essential in wound healing as observed in models of fetal wound healing. The tissue repair observed is also not limited to cutaneous but also tendon and myocardial regeneration. Clinical trials have also demonstrated that wounds treated with IL-10 have less scarring46.

In the case of leukocyte-rich aaPRP, it is hypothesized that the neutrophils present in the preparation may impede wound healing by releasing massive amounts of reactive oxygen species and nitric oxide. While these secretions are useful to prevent microbial infection and to clear debris from the wound, the high number of leukocytes may delay wound healing. One possible way to control the amount of neutrophils in leukocyte-rich aaPRP is to increase the number of macrophages in the preparation as they induce neutrophil apoptosis. Nevertheless, leukocyte-rich aaPRP has its own uses and benefits outside the context of DFU healing35.

Accessibility-wise, the study carried out by Linertová, .41 in Spain deemed that aaPRP treatment for DFUs is a cost-saving alternative in most cases when obtained manually as opposed to commercial kits when compared to usual care.

aaPRP is highly safe for human administration. Being autologous in nature, aaPRP has virtually no chance for rejection, cross-contamination, or disease transmission. Safety-wise, there is no rejection and it is minimally invasive. Interestingly, Karina, .40 formulated an aaPRP preparation protocol that is also suitable for systemic administration via intravenous infusion. The study also proved that aaPRP prepared with such a technique is devoid of leukocytes and platelets after the last stage of activation. Additionally, the aaPRP in the study was also administered through a blood transfusion set to eliminate any potential cellular debris. Out of the more than 600 patients, none reported any adverse effects related to the systemic aaPRP administration.

A double-blind randomized controlled trial (RCT) in 2021 by Yarahmadi found that aaPRP administration in a fibrin glue dressing (along with oral vitamin E and C supplementation) has the capability to help in DFU wound healing. This is evidenced by a significant wound size reduction and decrease in prooxidants compared to the control group39. This study results prove that aaPRP can decrease the immense oxidative stress brought about by hyperglycemia. On a side note, another RCT involving perineural aaPRP injection for DPN also showed an improvement in pain and numbness42. This encouraging result for improved peripheral nerve function may suggest that the benefits of aaPRP in DFU also extend beyond its antioxidative and anti-inflammatory properties.

Ahmed 37 also found that aaPRP in gel form is superior to a local antiseptic dressing in terms of healing rate and infection prevention in clean ulcers. In a previous canine study, aaPRP preparations have exhibited antimicrobial activity against methicillin-resistant 47. This antimicrobial property may be attributed to the presence of a chemokine (C-C motif) ligand (CCL)-3, CCL5, and chemokine (C-X-C motif) ligand (CXCL)-1. Along with CCL3, CCL5, and CXCL1, transforming growth factor-β1 is also present and reported to be bacteriostatic. Furthermore, synergistic activity between aaPRP and antibiotics have also been observed48.

However, a recent RCT concluded that a PRP dressing is no more efficacious than a placebo of normal saline in DFU managed with total-contact casting38. The results of said study contradicts an older RCT done in 2006 by Driver .36 which found that 81.3% patients in the DFU group treated with aaPRP gel saw wound resolution compared to 42.1% patients in the saline gel group. The difference in administration and the presence of a total-contact cast may play a role in the difference of results; this is to be explored in future studies.

Studies combining aaPRP to enhance the regenerative potential and graft survival rates of mesenchymal stem cells also exist. The combination of aaPRP with stromal vascular fraction (SVF) may be implemented as a non-animal source of growth factors to support stem cell proliferation and differentiation49. A murine study of wound healing has also procured promising results, with the group of rats treated with SVF and aaPRP having the most accelerated wound healing compared to aaPRP or SVF alone50. An RCT further confirmed this dynamic between aaPRP and stem cells. Histological analyses demonstrated improved neovascularization and graft survival in DFU wounds, although these results are not accompanied by a shortened wound healing time43.

The findings of this literature review have been summarized in Table 2.