Percutaneous penetration kinetics of lidocaine and prilocaine in two local anesthetic formulations assessed by in vivo microdialysis in pigs

Effect of Topical Analgesia on Desensitization Following 8% Topical Capsaicin Application

To prevent pain associated with 8% capsaicin application, pretreatment with local anesthetics, such as EMLA (eutectic mixture of lidocaine 2.5% and prilocaine 2.5%), is considered an option. However, there is contradicting evidence regarding the effects of local analgesia on capsaicin-induced desensitization. In session 1, 2 skin areas in each forearm of 24 healthy volunteers were randomized to 2-hour pretreatment with EMLA/placebo cream. After pretreatment, 8% capsaicin patches were applied for 3 hours in 1 placebo and 1 EMLA pretreated area, obtaining the following four areas: Capsaicin + EMLA, Capsaicin + Placebo, EMLA alone, and Placebo. Pain intensity scores were assessed during the 3-hour application of capsaicin. Warmth detection, heat pain sensitivity, and microvascular reactivity were measured after the removal of capsaicin. After 24 hours, in session 2, all tests were repeated followed by histamine application in each area to examine itch intensity and neurogenic flare. Overall, EMLA caused significant reductions in capsaicin-induced pain compared with placebo (P= .007) and enhanced the capsaicin-induced increase in superficial blood perfusion immediately after the 3-hour capsaicin application (P< .01). Regardless of pretreatment, capsaicin induced heat hyperalgesia immediately after the application (P< .001). Twenty-four hours post application, heat pain sensitivity was normalized. However, WDT increased significantly (P< .001). Capsaicin tended to reduce the itch intensity and significantly reduced the neurogenic flare (P< .05) induced by histamine compared with EMLA alone. The findings suggest that pretreatment with topical analgesic cream reduces application site pain without interfering with the 8% topical capsaicin-induced desensitization. PERSPECTIVE: Pretreatment with local anesthetic EMLA cream might be considered a good therapeutic option to reduce the pain associated with 8% capsaicin application currently used for treatment of neuropathic pain syndromes. This study also suggests the existence of a synergistic effect of capsaicin and EMLA on the process of neurogenic inflammation.

Bioglass Activated Albumin Hydrogels for Wound Healing

In this study, a novel Bioglass/albumin composite hydrogel with controllable injectability, good adhesiveness, and bioactivity, is developed by utilizing dual-functional bioactive ions released from Bioglass, which on one side controls the gelling time by creating an alkaline environment to regulate the cross-linking reaction between human serum albumin and succinimidyl succinate modified poly(ethylene glycol), and on the other side stimulates wound healing. The composite hydrogel exhibits adhesive property that is superior to clinically used fibrin and cyanoacrylate glues. The gelation time of the composite hydrogel could be regulated via changing the amounts of Bioglass which endows the hydrogel with good injectability. The in vivo experiment confirms that this composite hydrogel has good bioactivity to stimulate angiogenesis and enhance chronic wound healing. Moreover, for the first time, the concentrations of the bioactive ions released from the composite hydrogel in situ are quantified during wound healing using a microdialysis technique, and a correlation of the in vitro and in vivo concentration of ions released from the hydrogel is determined, which is extremely important for understanding the bioactivity mechanisms of Bioglass/bioceramic-based biomaterials and designing biomaterials for tissue regeneration.

Preclinical pilot study monitoring topical drug penetration and dermal bioavailability of a peptidase inhibitor from different galenic formulations into pig dermis, using cutaneous microdialysis

BACKGROUND

Cutaneous microdialysis (CM) is an ex vivo technique that allows study of tissue chemistry, including bioavailability of actual tissue concentration of unbound drug in the interstitial fluid of the body.

AIM

To test the penetration and dermal bioavailability of galenic formulations of the small-molecule IP10.C8, a dual-protease inhibitor of the dipeptidyl peptidase and aminopeptidase families.

METHODS

Using CM, we tested the penetration and dermal bioavailability of IP10.C8 into the dermis and subcutis of pigs, and determined the tissue concentration of IP10.C8 enzymatically, using an enzyme activity assay (substrate Gly-Pro-pNA) and high performance liquid chromatography.

RESULTS

Dermal bioavailability was enhanced by using microemulsion or the addition of the penetration enhancer oleic acid to a hydroxyethylcellulose (HEC) gel formulation. Dermal bioavailability was also enhanced when galenic formulations were prepared with higher pH (7.5 vs. 6.5) or higher drug concentration (5% vs. 1%) in HEC gel.

CONCLUSION

It seems possible, using CM for topical skin penetration testing in anaesthetized domestic pigs, to test the bioavailability of newly designed drugs. However, the experimental time is limited due to the anaesthesia, and is dependent on drug recovery. Validation of this technique for routine use is challenging, and more experiments are needed to validate this preclinical set-up.

Evaluation of a 13-hexyl-berberine hydrochloride topical gel formulation

13-hexyl-berberine hydrochloride (HB-13) is a derivative from berberine which finds widespread applications in the treatment of infectious pathogens including fungi, bacteria, parasites and viruses. As our continuing efforts for treatment of herpes simplex virus (HSV), we studied the topical delivery and safety of HB-13 in a gel formulation (0.5%) in a pig model. Our studies demonstrated the maximal HB-13 concentration was 2.51 µg/mL, which was more than the half maximal inhibitory concentration (IC50) as we previously reported. In addition, there was no sign of irritation or histological aberrance for stripped skin continuously applied with 0.5% HB-13 gel for 21 days. In conclusion, 0.5% HB-13 gel can achieve effective anti-HSV concentration in the dermis and it is safe to use.

Microdialysis Technique to Quantify Drug Concentration in Human Intervertebral Disks

No dependable method has yet been established to find time-dependent concentrations of substances injected into the intervertebral disk. This study investigated the feasibility of microdialysis in the measurement of local concentrations of a low-molecular weight drug in the human lumbar disk. A quasi-static experiment and a dynamic computer finite element simulation were used to study the spread of lidocaine in the lumbar disk. Fresh-frozen cadaveric lumbar motion segments were immersed in a 0.1% lidocaine HCl solution for 6 days prior to continuous microdialysis sampling for 30 min at the posterolateral annulus. Samples were collected every 10 min, for a total of three samples per probe. To maintain quasi-static conditions, where the output of lidocaine was equal to the diffusion rate, the microdialysis flow rate was set to 0.6 μl/min. The finite element model treated the disk as poroelastic tissue under compressive load and introduced 1 ml of 4% lidocaine into the nucleus pulposus. Higher microdialysis flow rates suffered from significant losses during consecutive recoveries. Relative recovery in the annulus at 0.6 μl/min was found to be 53.3%±16.2% of the initial solution. This was determined to be a result of low diffusivity of lidocaine through tissue. The FEA model predicted low diffusivity of lidocaine and slow transport to the posterolateral annulus if no fissures were present in the annulus. The results from in vitro experiments and computer simulations showed that while microdialysis can take reliable concentration measurements in the posterolateral annulus, probe placement near a fissure is critical if a measurement is to be made immediately following injection of a drug into the nucleus.

Formulation and evaluation of lidocaine lipid nanosystems for dermal delivery

The objective of the present investigation was to formulate solid lipid nanoparticles (SLN) and nanostructured lipid carriers (NLC) for improving the dermal delivery of a local anesthetic agent lidocaine (LID). SLN and NLC were characterized for particle size distribution, polydispersity index, entrapment efficiency, X-ray powder diffraction pattern (XRD), thermal behavior by differential scanning colorimeter (DSC) and surface morphology by transmission electron microscopy (TEM). LID-loaded SLN and NLC were formulated into hydrogels for topical application. The in vitro permeation profiles of LID SLN gel, LID NLC gel, and a marketed LID formulation (Xylocaine gel) were evaluated by using guinea pig skin. The in vivo efficacy of LID SLN gel, LID NLC gel, and a marketed LID formulation (Xylocaine gel) gel was evaluated on guinea pig using pinprick test. LID SLN showed a particle size of 78.1 nm with a polydispersity index of 0.556, whereas LID NLC showed a particle size of 72.8 nm with a polydispersity index of 0.463. The entrapment efficiency of LID in both SLN and NLC was 97% and 95.9%, respectively. The TEM studies revealed the almost spherical nature of LID SLN and NLC formulations. The XRD and DSC studies of LID SLN suggested amorphization of drug in the carrier system. The SLN formulation was stable with respect to particle size, polydispersity, and entrapment efficiency for 6 months at 40 degrees C/75% relative humidity (RH). Negligible leakage was observed for the NLC formulation when stored for 1 month at 40 degrees C/75% RH. In vitro permeation studies indicated that LID SLN gel and LID NLC gel significantly sustained the LID release compared to that of Xylocaine gel. The in vivo efficacy results supported the results of the in vitro permeation studies wherein the LID SLN gel and LID NLC gel resulted in fivefold and sixfold increase in duration of anesthesia, respectively, compared to that of Xylocaine gel.