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Interest in and use of topical analgesics has been increasing, presumably due to their potential utility for relief of acute and chronic pain. Topically applied agents with analgesic properties can target peripheral nociceptive pathways while minimizing absorption into the plasma that leads to potential systemic adverse effects.Clinical trials have found 5% lidocaine patches to be effective and well tolerated for the treatment of post-herpetic neuralgia (PHN) with a minimal risk of toxicity or drug-drug interactions. With this patch formulation, the penetration of lidocaine into the skin produces an analgesic effect without producing a complete sensory block. Use of topical lidocaine is supported by clinical practice guidelines, including first-line treatment by the American Academy of Neurology (guidelines retired 2018), the European Federation of Neurological Societies and second-line by the Canadian Pain Society.FDA approved 5% lidocaine patches in 1999, and a 1.8% topical lidocaine system in 2018 - both indicated for the treatment of pain secondary to PHN. The 1.8% system offers a more efficient delivery of lidocaine that is bioequivalent to 5% lidocaine patches, but with a 19-fold decrease in drug load (i.e., 36 mg versus 700 mg) as well as superior adhesion that allows the patch to maintain contact with the skin during the 12-h administration period.Although topical lidocaine formulations have advanced over time and play an important role in the treatment of PHN, a variety of other conditions that respond to topical lidocaine have been reported in the literature including PHN, lower back pain, carpal tunnel syndrome, diabetic peripheral neuropathy, and osteoarthritis joint pain. Other neuropathic or nociceptive pain syndromes may respond to topical lidocaine in select cases and warrant further study. Clinicians should consider local anesthetics and other topical agents as part of their multimodal treatments of acute and chronic pain.

Srinivas Nalamachu,1,2 Jeffrey Gudin3,4 1Mid America PolyClinic, Overland Park, KS, USA; 2Kansas City University of Medicine and Biosciences, Kansas City, MO, USA; 3Department of Anesthesiology and Pain Management, Englewood Hospital and Medical Center, Englewood, NJ, USA; 4Department of Anesthesiology and Perioperative Medicine, Rutgers New Jersey School of Medicine, Newark, NJ, USACorrespondence: Srinivas Nalamachu Tel +1 913-317-5300Fax +1 913-317-5301Email nalamachu@yahoo.comAbstract: Topical and transdermal formulations are a common means of pharmaceutical drug delivery. If a drug is able to penetrate transcutaneously, the skin is an ideal site for the delivery of medications for both local (topical) and systemic (transdermal) effects. The administration of analgesics through the skin poses several potential advantages to those administered orally including compliance, the ability to deliver a drug to a peripheral target site and more stable and sustained plasma levels. One method of drug delivery is with the use of patch formulations – also known as patch systems. Typically, transdermal patches deliver medications intended to reach the systemic circulation, whereas topical patches are designed to keep medication localized for targeted delivery in proximity to the application site. There are a variety of technologies and materials utilized in patches, as well as penetration and formulation enhancers that ultimately affect the performance, efficacy and safety of the patch system. The degree of adherence to the skin is also of critical importance in drug delivery. Patches that lift up or fall off before the prescribed time period may represent a therapeutic failure and must be replaced, increasing patch utilization and cost to the healthcare system or to the patient. The added risk from accidental exposure makes poor patch adhesion a safety issue as well. A variety of analgesics are currently available as patch formulations including local anesthetics, capsaicin, nonsteroidal anti-inflammatory drugs and opioids. This review will highlight each of those patch delivery systems and introduce newer patch technologies that lend towards improved adhesion and compliance. Understanding the designs, limitations and benefits of patch systems will allow clinicians to select between these therapies when appropriate for their patients.Keywords: topical, transdermal, lidocaine, capsaicin, patch adhesion

Jeffrey Gudin,1 Charles Argoff,2 Jeffrey Fudin,3 Emileigh Greuber,4 Kip Vought,4 Kalpana Patel,4 Sri Nalamachu5 1Department of Anesthesiology, Englewood Hospital and Medical Center, Englewood, NJ, USA; 2Department of Neurology, Albany Medical Center, Albany, NY, USA; 3Professional Practice, Albany College of Pharmacy and Health Sciences, Albany, NY, USA; 4Scilex Pharmaceuticals Inc., Palo Alto, CA, USA; 5Mid America PolyClinic, Overland Park, KS, USACorrespondence: Jeffrey FudinProfessional Practice, Albany College of Pharmacy and Health Sciences, Albany, NY, USATel +1 518-772-4100Fax +1 518-734-0288Email jeff@paindr.comPurpose: This study was designed to characterize drug delivery with lidocaine topical system 1.8% vs lidocaine patch 5% through 2 PK studies.Patients and Methods: Two Phase 1, single-center, open-label, randomized PK studies were performed in healthy adults. In Study 1, 56 subjects received a single intravenous bolus of 0.7 mg/kg of lidocaine as a lead-in to allow for the accurate determination of apparent dose of both products. After a 7-day washout period, subjects were randomized to receive either lidocaine topical system 1.8% or lidocaine patch 5% for 12 hours followed by another 7-day washout period, after which subjects crossed over to receive the other treatment for 12 hours. In Study 2, 54 subjects were randomized to receive either lidocaine topical system 1.8% or lidocaine patch 5% for 12 hours. After a 7-day washout period, subjects crossed over to receive the other treatment. Adhesion and skin irritation assessments were performed after application of the products in Study 2. In both studies, serial blood samples were collected to measure the plasma concentration of lidocaine after product application. Safety assessments and adverse events were monitored in both studies.Results: The comparative PK analysis demonstrated that the two products, despite their difference in drug load and strength, are bioequivalent. Both products were well tolerated. In Study 2, dermal response scores (skin tolerability after removal) were similar between lidocaine topical system 1.8% and lidocaine patch 5%, with a mean irritation score per patch < 1 (barely perceptible erythema), which is not considered to be clinically significant.Conclusion: Bioequivalence was demonstrated between lidocaine topical system 1.8% and lidocaine patch 5%. A comparison of the single-time adhesion scores at 12 hours in Study 2 favored lidocaine topical system 1.8% over lidocaine patch 5%. Both products were well tolerated as a single application in healthy adult human subjects.ClinicalTrials.gov: NCT04144192, NCT04149938.Keywords: postherpetic neuralgia, topical, lidocaine, pharmacokinetics