Topical drug formulations for prolonged corneal anesthesia

Resveratrol increases tear production and ocular pain after corneal abrasion in male, but not female, rats using a photorefractive keratectomy model

Photorefractive keratectomy (PRK) is an alternative to LASIK and can cause intense acute pain that is often not relieved by standard treatments. To assess potential therapeutics for this type of acute pain, appropriate preclinical models are needed. We describe a preclinical corneal abrasion rat model that simulates the initial stages of PRK surgery and demonstrates similar pain and tear dysfunction as seen clinically. We used both behavioral and homeostatic assays to determine the therapeutic potential of resveratrol on pain and tear production. Studies were conducted in male and female Sprague-Dawley rats. Heptanol was applied to one eye and the superficial corneal epithelium was removed, mimicking the abrasion used in PRK. Spontaneous pain was assessed with orbital tightening (OT) scores for 7 days. Topical resveratrol increased OT scores sex-specifically in abraded males, but not females, at 72 h and 1 week after abrasion. Resveratrol increased tear production in abraded males, with no effect in abraded females. There was no correlation between OT score at 1 week and tear production measurements, demonstrating no relationship between spontaneous ocular pain and tear dysfunction in this model. These findings demonstrate the usefulness of our corneal abrasion preclinical PRK model for the assessment of ocular pain therapeutics and indicate that topical resveratrol may not be useful for managing PRK-induced pain.

Prolonged Retrobulbar Local Anesthesia of the Cornea Does Not Cause Keratopathy in Mice

Purpose

Prolonged local anesthesia (PLA) of the cornea is currently assumed to cause neurotrophic keratitis and is strongly discouraged. We investigate whether PLA of the cornea per se causes neurotrophic keratitis.

Methods

PLA of the cornea was induced in 12 female albino BALB/c mice by retrobulbar injection of a polymeric prodrug (PGS-TTX) where the site 1 sodium channel blocker tetrodotoxin (TTX) was slowly released from the polymer polyglycerol sebacate. The duration and depth of corneal anesthesia was monitored by the Cochet-Bonnet esthesiometer. Corneal injury from PLA was assessed by slit lamp examination with 2% sodium fluorescein dye, histology, corneal nerve density by immunohistochemistry with anti-β III tubulin antibody and confocal microscopy, and corneal neurotrophin levels (substance P and neurokinin A) by an enzyme-linked immunosorbent assay. PLA was also induced by topical amitriptyline (80 mM), used as a positive control for local anesthetic-induced corneal injury. Frequent ocular lubrication was provided.

Results

Retrobulbar PGS-TTX resulted in complete corneal anesthesia lasting 50.1 ± 3.6 hours and mean time to complete resolution of block of 55.1 ± 3.6 hours with no keratopathy provided lubrication was provided. Topical 80 mM amitriptyline induced complete corneal anesthesia for 24 hours and developed keratopathy. There was no difference in the histology, levels of corneal neurotrophins, and corneal nerve density between the retrobulbar PGS-TTX group and normal cornea.

Conclusions

In the absence of topical toxicity or corneal exposure, PLA of the cornea per se does not cause keratitis.

Translational Relevance

PLA of the cornea could be highly beneficial in acute and chronic painful corneal conditions.

Effects of topical application of tramadol with/without dexmedetomidine and proparacaine on corneal sensitivity in rats

PURPOSE

To evaluate the corneal anesthetic effect following topical application of tramadol alone and in combination with dexmedetomidine, and compare it to proparacaine, in clinically healthy rats.

METHODS

A randomized, crossover study was performed. Twenty Wistar albino rats (n = 40 eyes) were used. Corneal touch threshold (CTT) measurements (in mm) were obtained using a Cochet-Bonnet aesthesiometer. CTT measurements were obtained at baseline, 1-min following application of the topical anesthetic agent, and repeated at 5-min intervals up to 75 min. The topical protocol involved 3 treatment conditions, separated by a 2-week washout period: proparacaine, tramadol alone, and tramadol in combination with dexmedetomidine.

RESULTS

CTT values were significantly decreased compared to baseline at each timepoint until completion of the 75-min evaluation in all treated eyes, regardless of the assigned treatment (p < 0.0083). With tramadol, complete corneal anesthesia (CTT = 0) was achieved within 1-5 min in 18 eyes and ranged from 5 to 25 min. Co-administration of dexmedetomidine to tramadol resulted in significantly increased CTT values from 5 to 20 min following topical application, compared to tramadol alone (p < 0.0083), and complete corneal anesthesia was achieved in only 14 out of 20 treated eyes.

CONCLUSION

Tramadol might be a useful alternative to topical anesthetic agents, providing a dose-related corneal anesthetic effect. Co-administration of dexmedetomidine does not potentiate its anesthetic effect. The underlying mechanism(s) of drug antagonism between tramadol and dexmedetomidine remains to be determined.

Neuronal delivery of nanoparticles via nerve fibres in the skin

Accessing the peripheral nervous system (PNS) by topically applied nanoparticles is a simple and novel approach with clinical applications in several PNS disorders. Skin is richly innervated by long peripheral axons that arise from cell bodies located distally within ganglia. In this study we attempt to target dorsal root ganglia (DRG) neurons, via their axons by topical application of lectin-functionalized gold nanoparticles (IB4-AuNP). In vitro, 140.2 ± 1.9 nm IB4-AuNP were found to bind both axons and cell bodies of DRG neurons, and AuNP applied at the axonal terminals were found to translocate to the cell bodies. Topical application of IB4-AuNP on rat hind-paw resulted in accumulation of three to fourfold higher AuNP in lumbar DRG than in contralateral control DRGs. Results from this study clearly suggest that topically applied nanoparticles with neurotropic targeting ligands can be utilized for delivering nanoparticles to neuronal cell bodies via axonal transport mechanisms.

KM-416, a novel phenoxyalkylaminoalkanol derivative with anticonvulsant properties exerts analgesic, local anesthetic, and antidepressant-like activities. Pharmacodynamic, pharmacokinetic, and forced degradation studies

Anticonvulsant drugs are used to treat a wide range of non-epileptic conditions, including chronic, neuropathic pain. We obtained a phenoxyalkylaminoalkanol derivative, KM-416 which had previously demonstrated a significant anticonvulsant activity and had also been shown to bind to 5-HT_1A, α₂-receptors and SERT and not to exhibit mutagenic properties. As KM-416 is a promising compound in our search for drug candidates, in the present study we further assessed its pharmacological profile (analgesic, local anesthetic, and antidepressant-like activities) accompanied with patch-clamp studies. Considering the importance of drug safety, its influence on the cardiovascular system was also evaluated. Moreover, KM-416 was subjected to forced degradation and pharmacokinetic studies to examine its stability and pharmacokinetic parameters. KM-416 revealed a significant antinociceptive activity in the tonic - the formalin test, neurogenic - the capsaicin test, and neuropathic pain model - streptozotocin-induced peripheral neuropathy. Moreover, it exerted a local anesthetic effect. In addition, KM-416 exhibited anti-depressant like activity. The results from the patch-clamp studies indicated that KM-416 can inhibit currents elicited by activation of NMDA receptors, while it also exhibited a voltage-dependent inhibition of Na⁺ currents. KM-416 did not influence ventricular depolarization and repolarization. Following oral administration, pharmacokinetics of KM-416 was characterized by a rapid absorption in the rat. The brain-to-plasma AUC ratio was 6.7, indicating that KM-416 was well distributed to brain. The forced degradation studies showed that KM-416 was very stable under stress conditions. All these features made KM-416 a promising drug candidate for further development against neuropathic pain and epilepsy.

Influence of Topical Anesthesia on Superficial Sensitivity: A Double-Blind, Randomized, Placebo-Controlled Study on 48 Healthy Subjects

BACKGROUND

Topical anesthetics are used in noninvasive transdermal anesthesia to decrease the superficial pain sensation threshold during dermatologic surgery. Combined pain relief and sensitivity loss can avoid discomfort during the surgery.

OBJECTIVE

The aim of this placebo-controlled study was to compare the efficacy of 3 commonly used topical agents by collating loss of sensitivity over time.

MATERIALS AND METHODS

Three topical anesthetic creams, a topical anti-inflammatory cream, and a moisturizing cream were applied on the left volar forearm of each of the 48 healthy Caucasian participants. Sensitivity was assessed with the dynamic 2-point discrimination and the Semmes-Weinstein test at 0, 60, 90, 120, 150, and 180 minutes after cream application.

RESULTS

After 180 minutes, benzocaine showed a significantly lower 2-point discrimination reduction than lidocaine alone and a lidocaine and prilocaine mixture. Sensory threshold measurements by the Semmes-Weinstein test after 60 minutes revealed a significantly higher effect with lidocaine alone and with the lidocaine and prilocaine mixture than with benzocaine.

CONCLUSION

The authors found a stronger skin sensitivity reduction by the eutectic lidocaine and prilocaine mixture and lidocaine alone compared with benzocaine. We suggest increased discomfort reduction in topical anesthetic supported dermatologic surgery by the eutectic mixture and lidocaine alone.

Prolonged Duration Topical Corneal Anesthesia With the Cationic Lidocaine Derivative QX-314

Purpose

Topical corneal local anesthetics are short acting and may impair corneal healing. In this study we compared corneal anesthesia and toxicity of topically applied N-ethyl lidocaine (QX-314) versus the conventional local anesthetic, proparacaine (PPC).

Methods

Various concentrations of QX-314 and 15 mM (0.5%) PPC were topically applied to rat corneas. Corneal anesthesia was assessed with a Cochet-Bonnet esthesiometer at predetermined time points. PC12 cells were exposed to the same solutions to assess cytotoxicity. Repeated topical corneal administration in rats was then used to assess for histologic evidence of toxicity. Finally, we created uniform corneal epithelial defects in rats and assessed the effect of repeated administration of these compounds on the defect healing rate.

Results

QX-314 (15 mM) and PPC (15 mM) caused similar total duration (114 ± 17 and 87 ± 16 minutes, respectively; P = 0.06) of anesthesia. The depth of anesthesia was similar between these low-dose groups at 15 minutes after application (1.8 ± 0.3- and 2.0 ± 0.8-cm filament lengths). QX-314 (100 mM) provided more prolonged corneal anesthesia (174 ± 13 minutes; P < 0.0001), with improved depth at 15 minutes (0.7 ± 0.3-cm filament length; P = 0.007). All tested concentrations of QX-314 demonstrated similar or less toxicity than 0.5% PPC.

Conclusions

Topical administration of QX-314 is effective for corneal anesthesia and demonstrates no histologic signs of local toxicity in a rodent model. In higher concentrations, QX-314 provides more than twofold the duration of anesthetic effect than does 0.5% PPC.

Translational Relevance

Our study reveals a clinically relevant compound providing prolonged duration topical corneal anesthesia.

Deep topical anesthesia with ropivacaine-soaked sponge for phacoemulsification

PURPOSE

To assess safety and efficacy of deep topical anesthesia with ropivacaine-soaked sponge compared with topical anesthesia with oxybuprocaine in patients undergoing phacoemulsification.

METHODS

This was a retrospective study where records of patients operated for cataract were evaluated. Patients using a visual analogue scale scored pain during surgery, and the surgeon on a questionnaire recorded ease of operation. Medical records were evaluated for patients who received topical anesthesia with multiple administrations of oxybuprocaine 0.4% or those who received deep topical anesthesia with a polyvinyl acetal sponge impregnated with ropivacaine 0.75% and positioned under the eyelid 30 min before surgery.

RESULTS

A total of one hundred patient records, equally divided in patients receiving deep topical anesthesia or topical anesthesia, were included. The visual analogue scale scores among the groups were statistically significant for a lower pain score in patients who received deep topical anesthesia with ropivacaine-soaked sponges (p = 0.0069). The average surgeon score was significantly higher for the deep topical anesthesia group indicating favorable ease of surgery (p = 0.0341). Six patients had major complications during surgery. No additional anesthesia was necessary to manage the complications in four patients in the deep anesthesia group, whereas propofol was used for the induction and maintenance of anesthesia in two patients in the topical anesthesia group.

CONCLUSIONS

Deep topical anesthesia with ropivacaine-soaked sponges performed as well as topical oxybuprocaine regarding safety and efficacy. It provided a lower patient pain score, favorable surgeon satisfaction, and long-lasting anesthesia.

Transtympanic Delivery of Local Anesthetics for Pain in Acute Otitis Media

Acute otitis media (AOM) commonly causes pain and distress in children. Existing analgesic ototopical drops have limited effectiveness due to the impermeable nature of the tympanic membrane. We developed a local drug delivery system to provide sustained pain relief in patients with AOM, achieved by applying a single dose of a hydrogel formulation onto the tympanic membrane. Successful drug delivery across intact tympanic membranes was demonstrated using the amino-amide anesthetic, bupivacaine, and a highly potent site 1 sodium channel blocker anesthetic, tetrodotoxin. The chemical permeation enhancers incorporated in the delivery system increased the permeability of the tympanic membrane to the anesthetics considerably. The drug levels measured using a previously developed ex vivo model reflect the potential for highly effective local anesthesia.

Addressing the Issue of Tetrodotoxin Targeting

This review is devoted to the medical application of tetrodotoxin (TTX), a potent non-protein specific blocker of voltage-gated sodium (NaV) channels. The selectivity of action, lack of affinity with the heart muscle NaV channels, and the inability to penetrate the blood–brain barrier make this toxin an attractive candidate for anesthetic and analgesic drug design. The efficacy of TTX was shown in neuropathic, acute and inflammatory pain models. The main emphasis of the review is on studies focused on the improvement of TTX efficacy and safety in conjunction with additional substances and drug delivery systems. A significant improvement in the effectiveness of the toxin was demonstrated when used in tandem with vasoconstrictors, local anesthetics and chemical permeation enhancers, with the best results obtained with the encapsulation of TTX in microparticles and liposomes conjugated to gold nanorods.

Long-acting liposomal corneal anesthetics

Eye drops producing long-acting ocular anesthesia would be desirable for corneal pain management. Here we present liposome-based formulations to achieve very long ocular anesthetic effect after a single eye drop instillation. The liposomes were functionalized with succinyl-Concanavalin A (sConA-Lip), which can bind corneal glycan moieties, to significantly prolong the dwell time of liposomes on the cornea. sConA-Lip were loaded with tetrodotoxin and dexmedetomidine (sConA-Lip/TD), and provided sustained release for both. A single topical instillation of sConA-Lip/TD on the cornea could achieve 105 min of complete analgesia and 608 min of partial analgesia, which was significantly longer than analgesia with proparacaine, tetrodotoxin/dexmedetomidine solution or unmodified liposomes containing tetrodotoxin and dexmedetomidine. sConA-Lip/TD were not cytotoxic in vitro to human corneal limbal epithelial cells or corneal keratocytes. Topical administration of sConA-Lip/TD provided prolonged corneal anesthesia without delaying corneal wound healing. Such a formulation may be useful for the management of acute surgical and nonsurgical corneal pain, or for treatment of other ocular surface diseases.

Getting Drugs Across Biological Barriers

The delivery of drugs to a target site frequently involves crossing biological barriers. The degree and nature of the impediment to flux, as well as the potential approaches to overcoming it, depend on the tissue, the drug, and numerous other factors. Here an overview of approaches that have been taken to crossing biological barriers is presented, with special attention to transdermal drug delivery. Technology and knowledge pertaining to addressing these issues in a variety of organs could have a significant clinical impact.

Topical anesthesia

Topical anesthetics are being widely used in numerous medical and surgical sub-specialties such as anesthesia, ophthalmology, otorhinolaryngology, dentistry, urology, and aesthetic surgery. They cause superficial loss of pain sensation after direct application. Their delivery and effectiveness can be enhanced by using free bases; by increasing the drug concentration, lowering the melting point; by using physical and chemical permeation enhancers and lipid delivery vesicles. Various topical anesthetic agents available for use are eutectic mixture of local anesthetics, ELA-max, lidocaine, epinephrine, tetracaine, bupivanor, 4% tetracaine, benzocaine, proparacaine, Betacaine-LA, topicaine, lidoderm, S-caine patch™ and local anesthetic peel. While using them, careful attention must be paid to their pharmacology, area and duration of application, age and weight of the patients and possible side-effects.

Corneal Anesthesia With Site 1 Sodium Channel Blockers and Dexmedetomidine

PURPOSE

Amino-amide or amino-ester local anesthetics, which are currently used for topical ocular anesthesia, are short acting and may delay corneal healing with long-term use. In contrast, site 1 sodium channel blockers (S1SCBs) are potent local anesthetics with minimal adverse tissue reaction. In this study, we examined topical local anesthesia with two S1SCBs, tetrodotoxin (TTX) or saxitoxin (STX) individually or in combination with α2-adrenergic receptor agonists (dexmedetomidine or clonidine), and compared them with the amino-ester ocular anesthetic proparacaine. The effect of test solutions on corneal healing was also studied.

METHODS

Solutions of TTX ± dexmedetomidine, TTX ± clonidine, STX ± dexmedetomidine, dexmedetomidine, or proparacaine were applied to the rat cornea. Tactile sensitivity was measured by recording the blink response to probing of the cornea with a Cochet-Bonnet esthesiometer. The duration of corneal anesthesia was calculated. Cytotoxicity from anesthetic solutions was measured in vitro. The effect on corneal healing was measured in vivo after corneal debridement followed by repeated drug administration.

RESULTS

Addition of dexmedetomidine to TTX or STX significantly prolonged corneal anesthesia beyond that of either drug alone, whereas clonidine did not. Tetrodotoxin or STX coadministered with dexmedetomidine resulted in two to three times longer corneal anesthesia than did proparacaine. S1SCB-dexmedetomidine formulations were not cytotoxic. Corneal healing was not delayed significantly by any of the test solutions.

CONCLUSIONS

Coadministration of S1SCBs with dexmedetomidine provided prolonged corneal anesthesia without delaying corneal wound healing. Such formulations may be useful for the management of acute surgical and nonsurgical corneal pain.

Corneal epithelial debridement for diagnosis and therapy of ocular surface disease

BACKGROUND

There is a wide range of ocular surface conditions that can be diagnosed or treated with epithelial debridement. The purpose of this study is to analyse the indications, instrumentation and outcomes of corneal epithelial debridement for anterior segment pathology.

METHODS

One hundred and thirty-three eyes of 129 patients with ocular surface disease were treated with epithelial debridement. Different methods were used, appropriate to the specific disease, aetiology and outcome measure.

RESULTS

The mean patient age was 56 years and 81 (60.9 per cent) were male. Sixty-two eyes (46.6 per cent) were undertaken for diagnosis and 71 (53.4 per cent) for therapy. Common indications for diagnostic corneal debridement included infective keratitis 48 (36.1 per cent), neoplasia 14 (10.5 per cent), while those for therapeutic corneal debridement included recurrent erosion 30 (22.5 per cent) and band keratopathy 16 (12.0 per cent). The most common post-operative complication was pain, occurring in all patients to some extent. Band keratopathy was seen to recur in two (12.5 per cent) of the treated eyes and there was a single relapse of recurrent erosion during the follow-up period.

CONCLUSION

The techniques presented can be undertaken in the office rather than a day procedural unit, improving time and cost-effectiveness for the ophthalmologist and patient.