A breakthrough zwitterionic polymer glides through the toughest skin barriers, delivering insulin deep into tissue and normalizing blood sugar, offering patients a painless alternative to daily injections.
Study: A skin-permeable polymer for non-invasive transdermal insulin delivery. Image credit: Me dia/Shutterstock.com
A recent study published in the journal Nature examines the use of the skin-permeable poly molecule[2-(N-oxide-N,N-dimethylamino)ethyl methacrylate] (OP) as an insulin delivery system, a key drug in the treatment of type 1 and many cases of type 2 diabetes mellitus.
Small molecule drugs are often designed to be absorbed through the skin. However, this has not been found feasible for large biomolecules such as proteins and peptides such as insulin.
Obstacles to insulin delivery
Insulin is usually given by intradermal injection. This method is painful, can cause fear of needles and skin complications, which are associated with poor patient compliance. No successful non-invasive insulin delivery technique has yet been reported.
Transdermal drug delivery offers many advantages, including improved patient compliance, convenience, increased concentration of active drug by avoiding denaturation, and reduced first-pass metabolism of the drug. Among the biggest challenges in this approach is getting past the stratum corneum (SC) of the skin.
The SC consists of dead and desiccated keratinocytes surrounded by a well-arranged fatty matrix. Together with the tight junctions of the epithelium in the epidermis and dermis, this is a barrier to drug penetration. Possible approaches include chemical penetration enhancers, electrical devices that force the drug to penetrate the skin, and ultrasound or jet injection instead of hypodermic needles, as well as microneedles. However, because they are invasive, they are associated with a higher risk of infection.
Cationic peptides can sometimes pass through the skin, bound to organic acids in the sebum and stratum corneum. However, this binding immobilizes them in the SC, preventing their deeper diffusion. Their only route is through hair follicles and sweat glands, which make up <0.1% of the skin's surface, meaning penetration is inefficient.
This prompted the current exploration of the new OP polymer. The extreme skin permeability of OP-I does not imply any change in the lipid sequence or structure of the skin. In contrast, molecular dynamics simulations revealed that OP-I was absorbed by stratum corneum fats faster than native insulin, rapidly diffusing through the lipids to reach the dermis and subcutaneous tissue.
This was characterized by a transition from its protonated cationic state (at pH 5 or lower) to a zwitterion upon crossing the skin (at neutral pH). This pH-dependent charge shift aligns with the skin’s acid-to-neutral gradient and is central to OP transport behavior.
This switch corresponds to the change in pH of the skin layers, going from superficial to deep. Upon topical application to the skin, OP accumulates in the acidic sebum and fatty acids contained in the fat layer of keratinized cells. In the deeper layers of the SC, which have a neutral pH, it becomes polyzitteric, thus favoring free diffusion by reducing electrostatic interactions with stratum corneum lipids.
The OP can thus pass quickly and smoothly through the skin into the blood and lymphatic vessels. OP and OP-insulin enter the systemic circulation mainly through the leaking lymphatic capillaries before reaching the bloodstream.
Study findings
Diffusion OP
The researchers applied fluorescently labeled OP to the skin surface of mice and minipigs (the latter’s skin closely resembles human skin) and monitored its passage through the skin using high-resolution imaging.
In mice, OP diffused into all layers of the skin within four hours of topical application, while control polyethylene glycol (PEG) remained on the skin surface. In the epidermis and dermis, OP-I moved by membrane-mediated diffusion without entering the cell. This involved rapid “jumping” across adjacent cell membranes rather than intracellular transport.
Further confirmation was obtained by visualizing the gold nanoparticles bound to OP within the lipid laminae of the fat layer of the mesenchymal cells. OP penetrates the skin with excellent efficiency, entering the bloodstream within 30 minutes. His concentration peaked about two hours later.
Coupled with OP insulin
Recombinant human insulin was then coupled to OP (OP-I), with pegylated insulin serving as a control, with a similar molecular mass of 5 kDa. OP-I had the same secondary structure as insulin. It also showed unchanged receptor binding and binding-dimension constants, indicating that it retained intact the receptor specificity and affinity of native insulin.
OP-I skin permeability was measured by the drop in blood glucose after topical application. Compared with unbound insulin, conjugated insulin produced the same reduction in blood glucose.
Thus, the study suggests that OP-I behaves similarly to insulin, binding to the insulin receptor with unchanged specificity and activating downstream pathways that result in glucose-lowering effects. OP-I had a longer half-life than insulin, possibly due to its zwitterionic nature that resists plasma protein binding and removal from the blood. This extension was modest (15 to 20 minutes vs. 5 to 10 minutes for natural insulin).
Modeling the skin penetration of OP-I over time using confocal laser scanning microscopy (CLSM) showed its uniform spread across the epidermis in half an hour. In contrast, native insulin and PEG-I remained on the skin surface. Thus, OP-I had the highest permeability coefficient among the three, approximately 4.5 and 9 times that of PEG-I and insulin, respectively.
Effects of OP on insulin and blood glucose
OP-I achieved comparable plasma levels to subcutaneous insulin within two hours. After that point, its levels were 60% to 600% higher than with insulin. The other two molecules did not affect insulin levels in the blood.
In mice with type 1 diabetes, OP-I normalized blood glucose levels. Once in the blood, OP-I was mainly taken up by the liver, lungs and kidneys, inducing insulin activity. Its activity was prolonged compared to subcutaneous native insulin, which was rapidly cleared from the bloodstream without significant accumulation in these tissues. OP-I regulated blood glucose levels in diabetic mice better than other treatments.
Similar findings were observed in minipigs, with OP-I entering the dermis and subcutaneous tissue four hours after topical application. It produced normal blood glucose levels within two hours and maintained them for 12 hours.
In particular, topical application of OP-I did not irritate the skin or cause inflammation. Repeated application to both mice and piglets did not induce structural changes in the stratum corneum or signs of immunoactivation.
To insulin without a needle
The skin-permeable polymer may enable non-invasive transdermal delivery of insulin, relieving diabetes patients of subcutaneous injections and potentially facilitating the patient-friendly use of other protein- and peptide-based therapies via transdermal delivery.
