Mikolaj Milewski

Current Aspects of Formulation Efforts and Pore Lifetime Related to Microneedle Treatment of Skin

Importance of the field: The efficacy of microneedles in the area of transdermal drug delivery is well documented. Multiple studies have shown that enhancement of skin permeation by means of the creation of microscopic pores in the stratum corneum can greatly improve the delivery rates of drugs. However, skin pretreatment with microneedles is not the only factor affecting drug transport rates. Other factors, including drug formulation and rate of micropore closure, are also important for optimizing delivery by this route. Areas covered in this review: This review aims to highlight work that has been done in these areas, with an emphasis on drug formulation parameters that affect transdermal flux. What the reader will gain: This review creates an appreciation for the many factors affecting microneedle-enhanced delivery. Most results clearly indicate that microneedle skin pretreatment by itself may have different effects on drug transport depending on the formulation used, and formulation characteristics have different effects on the transport through untreated skin and microneedle-treated skin. Several formulation approaches are reported to optimize microneedle-enhanced drug delivery, including co-solvent use, vesicular, nanoparticulate and gel systems. Take home message: In addition to well-established factors that affect microneedle-assisted delivery (geometry, type of microneedle, etc.), formulation and pore viability are also critical factors that must be considered.

In vitro permeation of a pegylated naltrexone prodrug across microneedle-treated skin

Microneedles (MN) are a useful tool for increasing skin permeability to xenobiotics. Previous research showed marked improvement in the percutaneous flux of naltrexone (NTX) hydrochloride by the use of MN skin pretreatment alone; however, for better therapeutic effect, further enhancement is desired. The goal of this in vitro study was to combine microneedle skin pretreatment with the use of a highly water-soluble PEGylated naltrexone prodrug (polyethyleneglycol-NTX, PEG-NTX) to investigate its transdermal transport at varying concentrations. Solubility and stability of the prodrug were investigated. In vitro diffusion experiments employing MN-treated minipig skin were used to evaluate the performance of the PEGylated prodrug. The results revealed substantial deviation from ideal behavior, with the flux through MN-treated skin having a nonlinear relationship to the prodrug concentration in the donor solution. While in the lower concentration range tested the prodrug flux increase was proportional to the concentration increase, at high concentrations it showed no such dependence. Accounting for the decrease in the effective prodrug diffusivity accompanying the increase in viscosity, as predicted by the Stokes-Einstein equation, provided a rationale for the observed flux values. Increasing the viscosity of the donor solution is hypothesized to afford a curvilinear permeation profile for the PEGylated NTX prodrug.

Diclofenac delays micropore closure following microneedle treatment in human subjects

Drugs absorbed poorly through the skin are commonly delivered via injection with a hypodermic needle, which is painful and increases the risk of transmitting infectious diseases. Microneedles (MNs) selectively and painlessly permeabilize the outermost skin layer, allowing otherwise skin-impermeable drugs to cross the skin through micron-sized pores and reach therapeutic concentrations. However, rapid healing of the micropores prevents further drug delivery, blunting the clinical utility of this unique transdermal technique. We present the first human study demonstrating that micropore lifetime can be extended following MN treatment. Subjects received one-time MN treatment and daily topical application of diclofenac sodium. Micropore closure was measured with impedance spectroscopy, and area under the admittance-time curve (AUC) was calculated. AUC was significantly higher at MN+diclofenac sodium sites vs. placebo, suggesting slower rates of micropore healing. Colorimetry measurements confirmed the absence of local erythema and irritation. This mechanistic human proof-of-concept study demonstrates that micropore lifetime can be prolonged with simple topical administration of a non-specific cyclooxygenase inhibitor, suggesting the involvement of subclinical inflammation in micropore healing. These results will allow for longer patch wear time with MN-enhanced delivery, thus increasing patient compliance and expanding the transdermal field to a wider variety of clinical conditions.

Vehicle Composition Influence on the Microneedle-Enhanced Transdermal Flux of Naltrexone Hydrochloride

ABSTRACTPurposeTransdermal delivery of drugs is often limited by formidable barrier properties of stratum corneum (SC). Microneedles (MN) enable creation of transient microchannels in the SC and bypass this barrier. Many reports have focused on the great effectiveness of MN in improving percutaneous flux values of a variety of drugs over a large molecular size spectrum. The objective of the present study is to evaluate the influence of formulation on MN-enhanced transdermal transport of naltrexone hydrochloride (NTX HCl).MethodsA series of in vitro experiments employing binary mixtures of propylene glycol (PG) and water as vehicle were used with either MN-treated or untreated skin. A simple model taking into account two parallel flux values through intact skin and microchannels was used to analyze data.ResultsTransdermal permeation of NTX HCl from different donor solutions indicated that PG-rich formulations greatly limited MN-enhanced transport but had a much smaller effect on transport through intact skin.ConclusionsDiffusion through the microchannel pathway seems to be donor viscosity-related and follows the relationship predicted by the Stokes-Einstein equation as shown by linear dependence of flux on diffusivity of NTX in donor solutions.

Estimation of maximum transdermal flux of nonionized xenobiotics from basic physicochemical determinants.

An ability to estimate the maximum flux of a xenobiotic across skin is desirable from the perspective of both drug delivery and toxicology. While there is an abundance of mathematical models describing the estimation of drug permeability coefficients, there are relatively few that focus on the maximum flux. This article reports and evaluates a simple and easy-to-use predictive model for the estimation of maximum transdermal flux of xenobiotics based on three common molecular descriptors: logarithm of octanol-water partition coefficient, molecular weight and melting point. The use of all three can be justified on the theoretical basis of their influence on the solute aqueous solubility and the partitioning into the stratum corneum lipid domain. The model explains 81% of the variability in the permeation data set composed of 208 entries and can be used to obtain a quick estimate of maximum transdermal flux when experimental data is not readily available.

Naltrexone Salt Selection for Enhanced Transdermal Permeation Through Microneedle-Treated Skin

The passive delivery rate of naltrexone (NTX) through intact skin is too slow to achieve therapeutic plasma levels in humans from a reasonably sized transdermal patch. A physical enhancement method--microneedles (MNs)--has been shown to afford a substantial increase in the percutaneous flux of NTX hydrochloride in vitro. However, for better therapeutic effect and decrease in the transdermal patch area, further enhancement is desired. The purpose of this study was to identify a NTX salt that would (1) provide elevated in vitro percutaneous drug transport across MN-treated skin as compared with that of the NTX hydrochloride and (2) prove nonirritating to the skin in vivo. The pH-solubility profiles of NTX salts were investigated with three drug salts showing improved solubility at physiologically relevant skin surface pH of 5.0. The skin-irritation potential of NTX glycolate and lactate gels was not greater than that of placebo gel in the guinea pig model. Additionally, in vitro diffusion studies indicated that NTX glycolate provides around 50% enhancement in the flux through MN-treated skin at the cost of doubling the drug concentration in the donor solution. Overall, a new NTX glycolate salt appears to be a promising candidate for MN-assisted transdermal drug delivery system.

Novel 3-O-pegylated carboxylate and 3-O-pegylated carbamate prodrugs of naltrexone for microneedle-enhanced transdermal delivery

A small library of novel 3-O-pegylated carboxylate prodrugs (4a-4b) and 3-O-pegylated carbamate prodrugs (9a-9b) of naltrexone were synthesized. The goal behind the design of these prodrugs was to investigate their potential for microneedle-enhanced transdermal delivery. All the synthesized 3-O-pegylated carboxylate prodrugs (4a-4b) and 3-O-pegylated carbamate prodrugs (9a-9b) of naltrexone were found to have adequate stability in a transdermal formulation and improved apparent solubility compared to naltrexone. Viscosity effects were postulated to be responsible for the observed non-linearity in the flux-concentration profile of these prodrugs.

Synthesis and in vitro stability of amino acid prodrugs of 6-β-naltrexol for microneedle-enhanced transdermal delivery

A small library of amino acid ester prodrugs of 6-β-naltrexol (NTXOL, 1) was prepared in order to investigate the candidacy of these prodrugs for microneedle-enhanced transdermal delivery. Six amino acid ester prodrugs were synthesized (6a-f). 6b, 6d, and 6 e were stable enough at skin pH (pH 5.0) to move forward to studies in 50% human plasma. The lead compound (6 e) exhibited the most rapid bioconversion to NTXOL in human plasma (t1/2 = 2.2 ± 0.1h).