Encapsulation of mepivacaine prolongs the analgesia provided by sciatic nerve blockade in mice

"Knowing It Before Blocking It," the ABCD of the Peripheral Nerves: Part B (Nerve Injury Types, Mechanisms, and Pathogenesis)

Selander emphatically said, "Handle these nerves with care," and those words still echo, conveying a loud and clear message that, however rare, peripheral nerve injury (PNI) remains a perturbing possibility that cannot be ignored. The unprecedented nerve injuries associated with peripheral nerve blocks (PNBs) can be most tormenting for the unfortunate patient and a nightmare for the anesthetist. Possible justifications for the seemingly infrequent occurrences of PNB-related PNIs include a lack of documentation/reporting, improper aftercare, or associated legal implications. Although they make up only a small portion of medicolegal claims, they are sometimes difficult to defend. The most common allegations are attributed to insufficient informed consent; preventable damage to a nerve(s); delay in diagnosis, referral, or treatment; misdiagnosis, and inappropriate treatment and follow-up care. Also, sufficient prospective studies or randomized trials have not been conducted, as exploring such nerve injuries (PNB-related) in living patients or volunteers may be impractical or unethical. Understanding the pathophysiology of various types of nerve injury is vital to dealing with them further. Processes like degeneration, regeneration, remyelination, and reinnervation can influence the findings of electrophysiological studies. Events occurring in such a process and their impact during the assessment determine the prognosis and the need for further interventions. This educational review describes various types of PNB-related nerve injuries and their associated pathophysiology.

DoE development of ionic gradient liposomes: A successful approach to improve encapsulation, prolong anesthesia and decrease the toxicity of etidocaine

Etidocaine (EDC) is a long-acting local anesthetic of the aminoamide family whose use was discontinued in 2008 for alleged toxicity issues. Ionic gradient liposomes (IGL) are nanostructured carriers for which an inner/outer gradient of ions increases drug upload. This work describes IGL_EDC, a formulation optimized by Design of Experiments, composed of hydrogenated soy phosphatidylcholine:cholesterol:EDC, and characterized by DLS, NTA, TEM/Cryo-TEM, DSC and ¹H NMR. The optimized IGL showed significant encapsulation efficiency (41 %), good shelf stability (180 days) and evidence of EDC interaction with the lipid bilayer (as seen by DSC and ¹H NMR results) that confirms its membrane permeation. In vitro (release kinetics and cytotoxicity) tests showed that the encapsulation of EDC into the IGL promoted sustained release for 24 h and decreased by 50 % the intrinsic toxicity of EDC to Schwann cells. In vivo IGL_EDC decreased the toxicity of EDC to Caenorhabditis elegans by 25 % and extended its anesthetic effect by one hour, after infiltrative administration, at clinically used (0.5 %) concentration, in rats. Thus, this novel drug delivery system is a promise for the possible reintroduction of EDC in clinics, aiming at the control of operative and postoperative pain.

The Influence of Diabetic Peripheral Neuropathy on the Duration of Sciatic Nerve Block with 1.3% Liposomal Bupivacaine and 0.25% Bupivacaine Hydrochloride in a Mouse Model

Little is known regarding the pharmacological properties of extended-release local anesthetics in the setting of diabetic peripheral neuropathy. We investigated and compared the duration of sciatic nerve block following administration of clinically relevant concentrations of liposomal bupivacaine (LB) and bupivacaine hydrochloride (BH) in diabetic mice with peripheral neuropathy. In this prospective, randomized, and double-blind study, twenty-four female C57BL/6J-OlaHsd mice were assigned to a streptozotocin-induced type 1 diabetes group and a control group without diabetes. The presence of peripheral neuropathy was established by assessing the duration of thermal latency of the plantar and tail-flick tests, following which both groups were subdivided into two subgroups in which 35 mg/kg of 1.31% LB and 7 mg/kg of 0.25% BH were respectively administered for sciatic nerve block. The average sensory block duration with BH was 106 min and 117.1 min in the control and diabetic groups, respectively. With LB, the average sensory block duration was 118 min in the control mice, while in mice with diabetic peripheral neuropathy, the average block duration was significantly longer and above the 270 min limit set in our study. Accordingly, sensory block duration was longer with LB compared to BH, and diabetic peripheral neuropathy significantly increased sciatic nerve block duration with LB.

Prolonged anesthesia and decreased toxicity of enantiomeric-excess bupivacaine loaded in ionic gradient liposomes

Bupivacaine is the most employed local anesthetic in surgical procedures, worldwide. Its systemic toxicity has directed the synthesis of the less toxic, S(-) enantiomer. This work describes a formulation of ionic gradient liposomes (IGL) containing _S75BVC, an enantiomeric excess mixture of 75% S(-) and 25% R(+) bupivacaine. IGL prepared with 250 mM (NH₄)₂SO₄ in the inner aqueous core of phosphatidylcholine and cholesterol (3:2 mol%) vesicles plus 0.5% _S75BVC showed average sizes of 312.5 ± 4.5 nm, low polydispersity (PDI < 0.18), negative zeta potentials (-14.2 ± 0.2 mV) and were stable for 360 days. The encapsulation efficiency achieved with IGL_S75BVC (%EE = 38.6%) was higher than with IGL prepared with racemic bupivacaine (IGL_RBVC, %EE = 28.3%). TEM images revealed spherical vesicles and µDSC analysis provided evidence on the interaction of the anesthetic with the lipid bilayer. Then, in vitro - release kinetics and cytotoxicity- and in vivo - toxic effects in Zebrafish and biochemical/histopathological analysis plus analgesia in Wistar rats - tests were performed. IGL_S75BVC exhibited negligible toxicity against Schwann cells and Zebrafish larvae, and it did not affect biochemical markers or the morphology of rat tissues (heart, brain, cerebellum, sciatic nerve). The in vitro release of _S75BVC from IGL was extended from 4 to 24 h, justifying the prolonged anesthetic effect measured in rats (~9 h). The advantages of IGL_S75BVC formulation over IGL_RBVC and plain bupivacaine formulations (prolonged anesthesia, preferential sensorial blockade, and no toxicity) confirm its potential for clinical use in surgical anesthesia.

Lidocaine-loaded reduced graphene oxide hydrogel for prolongation of effects of local anesthesia: In vitro and in vivo analyses

Lidocaine is widely used as a local anesthetic for alleviation of post-operative pain and for management of acute and chronic painful conditions. Although several approaches are currently used to prolong the duration of action, an effective strategy to achieve neural blockage for several hours remains to be identified. In this study, a lidocaine-loaded Pluronic® F68-reduced graphene oxide hydrogel was developed to achieve sustained release of lidocaine. Fourier transform infrared spectroscopy, X-ray diffraction, and Raman spectroscopy confirmed the synthesis of Pluronic® F68-reduced graphene oxide. Transmission electron microscopy showed wrinkled, flat nanosheets with micelles attached. The developed hydrogel showed desirable pH, viscosity, adhesiveness, hardness, and cohesiveness for topical application. The ex vivo release study demonstrated the ability of the Pluronic® F68-reduced graphene oxide hydrogel to prolong release up to 10 h, owing to the strong π-π interactions between the graphene oxide and the lidocaine. In comparison with a commercial lidocaine ointment, the developed graphene oxide hydrogel showed sustained anesthetic effect in the radiant heat tail flick test and sciatic nerve block model. Thus, this study demonstrates the potential of using Pluronic® F68-reduced graphene oxide nanocarriers to realize prolonged effects of local anesthesia for effective pain management.

Safety and efficacy concerns of modern strategies of local anesthetics delivery

In the last few decades, several formulations have evolved to realize better efficacy of administered anesthesia. These innovative formulations have facilitated surgeons to perform operations under purely local anesthesia, which provides extra protection and comfort to patients. Ease of delivery of local anesthesia is the need of the current generation, because some of the standard procedures are performed without the use of any sedative agent. Therefore, we are presenting here the various approaches of administration of local anesthetics by the surgeons. To construct a comprehensive report on various methods of anesthesia, we followed a systematic literature search of bibliographic databases of published articles recently in the international journals and publishers of repute. A comprehensive study of several reports of the field indicates that there are significant progresses towards developing novel formulations of anesthesia drugs as well as strategies of delivery. Among formulations, nanoparticle-based delivery approaches, including polymeric, liposomal, and micellar structures, have offered the much needed efficacy with low toxicity. Therefore, several of such techniques are at various stages of clinical trials. Nanotechnology-based delivery approaches have significantly emerged in recent past due to the low systemic toxicity and better efficacy of the nonconventional local anesthetics. The other methods of local anesthesia delivery such as transdermal, magnetophoresis, electrophoresis, and iontophoresis are frequently used due to them being minimally invasive and locally effective. Therefore, the combination of the nanotechnological methods with above mentioned techniques would significantly enhance the overall process of local anesthesia delivery and efficacy.

Mepivacaine reduces calcium transients in isolated murine ventricular cardiomyocytes

Background

The potential mechanism of mepivacaine’s myocardial depressant effect observed in papillary muscle has not yet been investigated at cellular level. Therefore, we evaluated mepivacaine’s effects on Ca²⁺ transient in isolated adult mouse cardiomyocytes.

Methods

Single ventricular myocytes were enzymatically isolated from wild-type C57Bl/6 mice and loaded with 10 μM fluorescent Ca²⁺ indicator Fluo-4-AM to record intracellular Ca²⁺ transients upon electrical stimulation. The mepivacaine effects at half-maximal inhibitory concentration (IC₅₀) was determined on calibrated cardiomyocytes’ Ca²⁺ transients by non-parametric statistical analyses on biophysical parameters. Combination of mepivacaine with NCX blockers ORM-10103 or NiCl₂ were used to test a possible mechanism to explain mepivacaine-induced Ca²⁺ transients’ reduction.

Results

A significant inhibition at mepivacaine’s IC₅₀ (50 μM) on Ca²⁺ transients was measured in biophysical parameters such as peak (control: 528.6 ± 73.61 nM vs mepivacaine: 130.9 ± 15.63 nM; p < 0.05), peak area (control: 401.7 ± 63.09 nM*s vs mepivacaine: 72.14 ± 10.46 nM*s; p < 0.05), slope (control: 7699 ± 1110 nM/s vs mepivacaine: 1686 ± 226.6 nM/s; p < 0.05), time to peak (control: 107.9 ± 8.967 ms vs mepivacaine: 83.61 ± 7.650 ms; p < 0.05) and D₅₀ (control: 457.1 ± 47.16 ms vs mepivacaine: 284.5 ± 22.71 ms; p < 0.05). Combination of mepivacaine with NCX blockers ORM-10103 or NiCl₂ showed a significant increase in the baseline of [Ca²⁺] and arrhythmic activity upon electrical stimulation.

Conclusion

At cellular level, mepivacaine blocks Na⁺ channels, enhancing the reverse mode activity of NCX, leading to a significant reduction of Ca²⁺ transients. These results suggest a new mechanism for the mepivacaine-reduction contractility effect.

Anaesthetic benefits of a ternary drug delivery system (Ropivacaine-in-Cyclodextrin-in-Liposomes): in-vitro and in-vivo evaluation

OBJECTIVES

To evaluate whether a ternary system composed of hydroxypropyl-β-cyclodextrin (HP-βCD) further encapsulated into egg phosphatidylcholine liposomes (LUV) could prolong the action and reduce the toxicity of ropivacaine (RVC).

METHODS

Dynamic light scattering and NMR were used to characterize the inclusion complex (RVC : HP-βCD), liposomal (RVC : LUV) and ternary (LUV : RVC : HP-βCD) systems containing 0.25% RVC. Their encapsulation efficiency, release kinetics, in-vitro cytotoxicity and in-vivo anaesthetic effect (paw-withdraw tests in mice) were also evaluated.

KEY FINDINGS

1 : 1 RVC : HP-βCD inclusion complex was encapsulated in liposomes (220.2 ± 20.3 nm size, polydispersity <0.25, zeta potentials = -31.7 ± 1.4 mV). NMR (diffusion-ordered spectroscopy (DOSY)) revealed stronger anaesthetic binding to LUV : RVC : HP-βCD (K_a  = 342 m^-1 ) than to RVC : HP-βCD (K_a  = 128 m^-1 ) or liposomal formulation (K_a  = 22 m^-1 ). The formulations promoted in-vitro sustained drug release and partially reverted the cytotoxicity of RVC against 3T3 fibroblasts in the profile: LUV : RVC : HP-βCD ≥ RVC : HP-βCD > RVC : LUV. Accordingly, in-vivo sensory block of free RVC (180 min) was prolonged ca. 1.7 times with the ternary system and RVC : HP-βCD (300 min) and 1.3 times with RVC : LUV (240 min).

CONCLUSIONS

These results confirm the suitability of this double-carrier system in clinical practice, to decrease the toxicity and prolong the anaesthesia time evoked by RVC.

Liposomal-based lidocaine formulation for the improvement of infiltrative buccal anaesthesia

This study describes the encapsulation of the local anaesthetic lidocaine (LDC) in large unilamellar liposomes (LUV) prepared in a scalable procedure, with hydrogenated soybean phosphatidylcholine, cholesterol and mannitol. Structural properties of the liposomes were assessed by dynamic light scattering, nanoparticle tracking analysis and transmission electron microscopy. A modified, two-compartment Franz-cell system was used to evaluate the release kinetics of LDC from the liposomes. The in vivo anaesthetic effect of liposomal LDC 2% (LUV_LDC) was compared to LDC 2% solution without (LDC_PLAIN) or with the vasoconstrictor epinephrine (1:100 000) (LDC_VASO), in rat infraorbital nerve blockade model. The structural characterization revealed liposomes with spherical shape, average size distribution of 250 nm and low polydispersity even after LDC incorporation. Zeta potential laid around -30 mV and the number of suspended liposomal particles was in the range of 10¹² vesicles/mL. Also the addition of cryoprotectant (mannitol) did not provoke structural changes in liposomes properties. In vitro release profile of LDC from LUV fits well with a biexponential model, in which the LDC encapsulated (EE% = 24%) was responsible for an increase of 67% in the release time in relation to LDC_PLAIN (p < 0.05). Also, the liposomal formulation prolonged the sensorial nervous blockade duration (∼70 min), in comparison with LDC_PLAIN (45 min), but less than LDC_VASO (130 min). In this context, this study showed that the liposomal formulations prepared by scalable procedure were suitable to promote longer and safer buccal anaesthesia, avoiding side effects of the use of vasoconstrictors.

Adjunct medications for peripheral and neuraxial anesthesia

Regional and neuraxial anesthesia can provide a safer perioperative experience, greater satisfaction, reduced opioid consumption, and reduction of pain, while minimizing side effects. Ultrasound technology has aided clinicians in depositing local anesthetic medication in precise proximity to targeted peripheral nerves. There are a plethora of adjuvants that have been utilized to prolong local anesthetic actions and enhance effects in peripheral nerve blocks. This manuscript describes the current state of the use of adjuncts, e.g., dexmedetomidine, dexamethasone, clonidine, epinephrine, etc., in regional anesthesia. Additionally, evidence behind dosing and block prolongation is summarized along with patient outcomes, adverse effects, and future directions.

Adjuvants to local anesthetics: Current understanding and future trends

Although beneficial in acute and chronic pain management, the use of local anaesthetics is limited by its duration of action and the dose dependent adverse effects on the cardiac and central nervous system. Adjuvants or additives are often used with local anaesthetics for its synergistic effect by prolonging the duration of sensory-motor block and limiting the cumulative dose requirement of local anaesthetics. The armamentarium of local anesthetic adjuvants have evolved over time from classical opioids to a wide array of drugs spanning several groups and varying mechanisms of action. A large array of opioids ranging from morphine, fentanyl and sufentanyl to hydromorphone, buprenorphine and tramadol has been used with varying success. However, their use has been limited by their adverse effect like respiratory depression, nausea, vomiting and pruritus, especially with its neuraxial use. Epinephrine potentiates the local anesthetics by its antinociceptive properties mediated by alpha-2 adrenoreceptor activation along with its vasoconstrictive properties limiting the systemic absorption of local anesthetics. Alpha 2 adrenoreceptor antagonists like clonidine and dexmedetomidine are one of the most widely used class of local anesthetic adjuvants. Other drugs like steroids (dexamethasone), anti-inflammatory agents (parecoxib and lornoxicam), midazolam, ketamine, magnesium sulfate and neostigmine have also been used with mixed success. The concern regarding the safety profile of these adjuvants is due to its potential neurotoxicity and neurological complications which necessitate further research in this direction. Current research is directed towards a search for agents and techniques which would prolong local anaesthetic action without its deleterious effects. This includes novel approaches like use of charged molecules to produce local anaesthetic action (tonicaine and n butyl tetracaine), new age delivery mechanisms for prolonged bioavailability (liposomal, microspheres and cyclodextrin systems) and further studies with other drugs (adenosine, neuromuscular blockers, dextrans).

From micro- to nanostructured implantable device for local anesthetic delivery

Local anesthetics block the transmission of painful stimuli to the brain by acting on ion channels of nociceptor fibers, and find application in the management of acute and chronic pain. Despite the key role they play in modern medicine, their cardio and neurotoxicity (together with their short half-life) stress the need for developing implantable devices for tailored local drug release, with the aim of counterbalancing their side effects and prolonging their pharmacological activity. This review discusses the evolution of the physical forms of local anesthetic delivery systems during the past decades. Depending on the use of different biocompatible materials (degradable polyesters, thermosensitive hydrogels, and liposomes and hydrogels from natural polymers) and manufacturing processes, these systems can be classified as films or micro- or nanostructured devices. We analyze and summarize the production techniques according to this classification, focusing on their relative advantages and disadvantages. The most relevant trend reported in this work highlights the effort of moving from microstructured to nanostructured systems, with the aim of reaching a scale comparable to the biological environment. Improved intracellular penetration compared to microstructured systems, indeed, provides specific drug absorption into the targeted tissue and can lead to an enhancement of its bioavailability and retention time. Nanostructured systems are realized by the modification of existing manufacturing processes (interfacial deposition and nanoprecipitation for degradable polyester particles and high- or low-temperature homogenization for liposomes) or development of novel strategies (electrospun matrices and nanogels). The high surface-to-volume ratio that characterizes nanostructured devices often leads to a burst drug release. This drawback needs to be addressed to fully exploit the advantage of the interaction between the target tissues and the drug: possible strategies could involve specific binding between the drug and the material chosen for the device, and a multiscale approach to reach a tailored, prolonged drug release.

Comparison of liposomal and 2-hydroxypropyl-β-cyclodextrin–lidocaine on cell viability and inflammatory response in human keratinocytes and gingival fibroblasts

Objectives

The aim of this study was to observe the effect multilamellar liposomes (MLV) and 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) in the in-vitro effects of lidocaine in cell viability, pro-inflammatory cytokines and prostaglandin E2 release of both human keratinocytes (HaCaT) and gingival fibroblasts (HGF) cells.

Methods

HaCaT and HGF cells were exposed to lidocaine 100–1 μm in plain, MLV and HP-β-CD formulations for 6 h or 24 h. The formulation effects in cell viability were measured by XTT assay and by fluorescent labelling. Cytokines (IL-8, IL-6 and TNF-α) and PGE2 release were quantified by ELISA.

Key findings

MLV and HP-β-CD formulations did not affect the HaCaT viability, which was significantly decreased by plain lidocaine after 24 h of exposure. Both drug carriers increased all cytokines released by HGF after 24-h exposure, and none of the carriers was able to reduce the PGE2 release induced by lidocaine.

Conclusion

The effect of drug carrier in the lidocaine effects was dependent on the cell type, concentration and time of exposure. MLV and HP-β-CD showed benefits in improving cell viability; however, both of them showed a tendency to increase cytokine release when compared to the plain solution.

Facile fabrication of polyanhydride/anesthetic nanoparticles with tunable release kinetics

This work illustrates a two-step strategy for the fabrication of polymer/drug nanoparticles. Utilizing solvent/non-solvent precipitation and gaseous basification, composite nanoparticles with 0-100% drug loadings are fabricated. Drug release kinetics are dictated by nanoparticle composition allowing future tuning for therapeutic applications.

Extended release formulations for local anaesthetic agents

Systemic toxicity through overdose of local anaesthetic agents is a real concern. By encapsulating local anaesthetics in biodegradable carriers to produce a system for prolonged release, their duration of action can be extended. This encapsulation should also improve the safety profile of the local anaesthetic as it is released at a slower rate. Work with naturally occurring local anaestheticss has also shown promise in the area of reducing systemic and neurotoxicity. Extended duration local anaesthetic formulations in current development or clinical use include liposomes, hydrophobic based polymer particles such as Poly(lactic-co-glycolic acid) microspheres, pasty injectable and solid polymers like Poly(sebacic-co-ricinoleic acid) P(SA:RA) and their combination with synthetic and natural local anaesthetic. Their duration of action, rationale and limitations are reviewed. Direct comparison of the different agents is limited by their chemical properties, the drug doses encapsulated and the details of in vivo models described.

Pharmacokinetic study of liposome-encapsulated and plain mepivacaine formulations injected intra-orally in volunteers

Objectives

The pharmacokinetics of commercial and liposome-encapsulated mepivacaine (MVC) injected intra-orally in healthy volunteers was studied.

Methods

In this double blind, randomized cross-over study, 15 volunteers received, at four different sessions, 1.8 ml of the following formulations: 2% MVC with 1 : 100 000 epinephrine (MVC2%EPI), 3% MVC (MVC3%), 2% and 3% liposome-encapsulated MVC (MVC2%LUV and MVC3%LUV). Blood samples were collected pre dose (0 min) and at 15, 30, 45, 60, 90, 120, 180, 240, 300, 360 min after injections. Liquid chromatography-tandem mass spectrometry was used to quantify plasma MVC concentrations.

Results

Pharmacokinetic analysis showed that the maximum plasma concentration (Cmax) and the areas under the curves (AUC0–360 and AUC0–∞) after MVC2%LUV and MVC2%EPI injections were smaller (P &lt; 0.05) than the equivalent figures for MVC3% and MVC3%LUV. The time to maximum plasma concentration (Tmax) and the half-life of elimination (t½beta) obtained after the treatment with MVC2%LUV, MVC2%EPI, MVC3% and MVC3%LUV presented no statistically significant differences (P &gt; 0.05). Cmax, AUC0–360 and AUC0-∞ after injection of the 2% formulations (MVC2%LUV and MVC2%EPI) did not exhibit statistically significant differences (P &gt; 0.05). The pharmacokinetics of MVC2%LUV were comparable to the pharmacokinetics of MVC2%EPI.

Conclusion

The liposomal formulation of 2% MVC exhibits similar systemic absorption to the local anesthetic with vasoconstrictor.

Controlled release system for ametryn using polymer microspheres: preparation, characterization and release kinetics in water

The purpose of this work was to develop a modified release system for the herbicide ametryn by encapsulating the active substance in biodegradable polymer microparticles produced using the polymers poly(hydroxybutyrate) (PHB) or poly(hydroxybutyrate-valerate) (PHBV), in order to both improve the herbicidal action and reduce environmental toxicity. PHB or PHBV microparticles containing ametryn were prepared and the efficiencies of herbicide association and loading were evaluated, presenting similar values of approximately 40%. The microparticles were characterized by scanning electron microscopy (SEM), which showed that the average sizes of the PHB and PHBV microparticles were 5.92±0.74 μm and 5.63±0.68 μm, respectively. The ametryn release profile was modified when it was encapsulated in the microparticles, with slower and more sustained release compared to the release profile of pure ametryn. When ametryn was associated with the PHB and PHBV microparticles, the amount of herbicide released in the same period of time was significantly reduced, declining to 75% and 87%, respectively. For both types of microparticle (PHB and PHBV) the release of ametryn was by diffusion processes due to anomalous transport (governed by diffusion and relaxation of the polymer chains), which did not follow Fick's laws of diffusion. The results presented in this paper are promising, in view of the successful encapsulation of ametryn in PHB or PHBV polymer microparticles, and indications that this system may help reduce the impacts caused by the herbicide, making it an environmentally safer alternative.

Review of prolonged local anesthetic action

IMPORTANCE OF THE FIELD

Pain following surgery is often treated by local anesthetic agents. Duration of the analgesia can be extended safely following administration of encapsulated large doses of local anesthetic agents.

AREAS COVERED IN THIS REVIEW

This review considers formulations used for encapsulation of local anesthetic agents for prolonged anesthesia effect. All studies describing encapsulation of a commercial local anesthetic agent for providing prolonged analgesia were considered using the NCBI Medline site. of local anesthetic, prolonged anesthesia, polymers and liposomes were entered in order to retrieve appropriate articles and reviews from 1966 to 2010, with emphasis on the last 10 years. Reference pages were searched manually for other relevant articles. The topics covered include an overview of local anesthetic agents and a review of local anesthetic carrier agents, with emphasis on liposomes and polymer carriers. Articles were limited to the English language.

WHAT THE READER WILL GAIN

The current research areas for prolongation of local anesthetic effect are evaluated, along with their limitations. Each topic has been summarized, and the review has attempted to cover all current laboratory and clinical studies in a simple manner that should also be useful for readers without a pharmacology background. The direction of research is promising and exciting, and this review should be a useful up-to-date reference.

TAKE HOME MESSAGE

Many formulations including polymer and liposome carriers have facilitated prolonged local anesthetic action for several days, although few clinical studies have been performed. This field promises a safe way to deliver local anesthetics for effect far beyond that of commercially available agents, with potential cost and health benefits for patients suffering chronic or postoperative pain.

Liposomal encapsulation improves the duration of soft tissue anesthesia but does not induce pulpal anesthesia

STUDY OBJECTIVE

To compare the topical and the pulpal anesthesia efficacy of liposomal and plain benzocaine formulations.

DESIGN

Double-blinded, randomized crossover study.

SETTING

University ambulatory dental center.

PATIENTS

30 ASA physical status I volunteers.

INTERVENTIONS

Volunteers received, in three different sessions, topical application of liposome-encapsulated 10% benzocaine (LB10), 10% benzocaine gel (B10), and 20% benzocaine gel (B20) in the right maxillary canine mucobuccal fold.

MEASUREMENTS

Pain associated with the needle insertion was rated by visual analog scale (VAS) and the duration of topical anesthesia was recorded. Pulpal anesthesia was evaluated using an electric pulp tester.

MAIN RESULTS

VAS values (median, 1st - 3rd quartiles) were 17 cm (11 - 25), 14 cm (3 - 22), and 21 cm (9 - 21) for B10, LB10, and B20, respectively. No differences were noted among the groups (Friedman test; P = 0.58). Soft tissue anesthesia was also not different. The LB10 [10 (8 - 12) min] showed longer soft tissue anesthesia (Friedman test; P < 0.01) than the other agents [B10 = 8 (5 - 10) min, and B20 = 7 (6 - 9) min]. None of the topical benzocaine formulations tested induced pulpal anesthesia.

CONCLUSIONS

The encapsulation of benzocaine into liposome increased the duration of soft tissue anesthesia. However, it did not induce pulpal anesthesia.

Pharmacological and local toxicity studies of a liposomal formulation for the novel local anaesthetic ropivacaine

This study reports an investigation of the pharmacological activity, cytotoxicity and local effects of a liposomal formulation of the novel local anaesthetic ropivacaine (RVC) compared with its plain solution. RVC was encapsulated into large unilamellar vesicles (LUVs) composed of egg phosphatidylcholine, cholesterol and α-tocopherol (4:3:0.07, mole%). Particle size, partition coefficient determination and in-vitro release studies were used to characterize the encapsulation process. Cytotoxicity was evaluated by the tetrazolium reduction test using sciatic nerve Schwann cells in culture. Local anaesthetic activity was assessed by mouse sciatic and rat infraorbital nerve blockades. Histological analysis was performed to verify the myotoxic effects evoked by RVC formulations. Plain (RVCPLAIN) and liposomal RVC (RVCLUV) samples were tested at 0.125%, 0.25% and 0.5% concentrations. Vesicle size distribution showed liposomal populations of 370 and 130 nm (85 and 15%, respectively), without changes after RVC encapsulation. The partition coefficient value was 132 ± 26 and in-vitro release assays revealed a decrease in RVC release rate (1.5 fold, P &lt; 0.001) from liposomes. RVCLUV presented reduced cytotoxicity (P &lt; 0.001) when compared with RVCPLAIN. Treatment with RVCLUV increased the duration (P &lt; 0.001) and intensity of the analgesic effects either on sciatic nerve blockade (1.4–1.6 fold) and infraorbital nerve blockade tests (1.5 fold), in relation to RVCPLAIN. Regarding histological analysis, no morphological tissue changes were detected in the area of injection and sparse inflammatory cells were observed in only one of the animals treated with RVCPLAIN or RVCluv at 0.5%. Despite the differences between these preclinical studies and clinical conditions, we suggest RVCLUV as a potential new formulation, since RVC is a new and safe local anaesthetic agent.

Prolonged duration local anesthesia with minimal toxicity

Injectable local anesthetics that would last for many days could have a marked impact on periprocedural care and pain management. Formulations have often been limited in duration of action, or by systemic toxicity, local tissue toxicity from local anesthetics, and inflammation. To address those issues, we developed liposomal formulations of saxitoxin (STX), a compound with ultrapotent local anesthetic properties but little or no cytotoxicity. In vitro, the release of bupivacaine and STX from liposomes depended on the lipid composition and on whether dexamethasone was incorporated. In cell culture, bupivacaine, but not STX, was myotoxic (to C2C12 cells) and neurotoxic (to PC12 cells) in a concentration- and time-dependent manner. Liposomal formulations containing combinations of the above compounds produced sciatic nerve blockade lasting up to 7.5 days (with STX + dexamethasone liposomes) in male Sprague-Dawley rats. Systemic toxicity only occurred where high loadings of dexamethasone increased the release of liposomal STX. Mild myotoxicity was only seen in formulations containing bupivacaine. There was no nerve injury on Epon-embedded sections, and these liposomes did not up-regulate the expression of 4 genes associated with nerve injury in the dorsal root ganglia. These results suggest that controlled release of STX and similar compounds can provide very prolonged nerve blocks with minimal systemic and local toxicity.

Stability and local toxicity evaluation of a liposomal prilocaine formulation

This study reports a physicochemical stability evaluation of a previously reported liposomal prilocaine (PLC(LUV)) formulation (Cereda et al. J. Pharm. Pharmaceut. Sci. 7:235, 2004) before and after steam sterilization as well as its local toxicity evaluation. Prilocaine (PLC) was encapsulated into extruded unilamellar liposomes (LUVs) composed by egg phosphatidylcholine:cholesterol:alfa-tocopherol (4:3:0.07, mole %). Laser light-scattering analysis (p > 0.05) and thiobarbituric acid reaction (p > 0.05) were used to evaluate the liposomes physical (size) and chemical (oxidation) stability, respectively. The prilocaine chemical stability was followed by (1)H-nuclear magnetic resonance. These tests detected no differences on the physicochemical stability of PLC or PLC(LUV), sterilized or not, up to 30 days after preparation (p > 0.05). Finally, the paw edema test and histological analysis of rat oral mucosa were used to assess the possible inflammatory effects of PLC(LUV). PLC(LUV) did not evoke rat paw edema (p > 0.05), and no significant differences were found in histological analysis, when compared to the control groups (p > 0.05). The present work shows that PLC(LUV) is stable for a 30-day period and did not induce significant inflammatory effects both in the paw edema test and in histological analysis, giving supporting evidence for its safety and possible clinical use in dentistry.

Hydroxymethylnitrofurazone:dimethyl-beta-cyclodextrin inclusion complex: a physical-chemistry characterization

Hydroxymethylnitrofurazone (NFOH) is active against Trypanosoma cruzi; however, its low solubility and high toxicity precludes its current use in treatment of parasitosis. Cyclodextrin can be used as a drug carrier system, as it is able to form inclusion (host-guest) complexes with a wide variety of organic (guest) molecules. Several reports have shown the interesting use of modified beta-cyclodextrins in pharmaceutical formulation, to improve the bioavailability of drugs and to decrease their toxicity. The aim of this work was to characterize inclusion complexes formed between NFOH and dimethyl-beta-cyclodextrin (DM-beta-CD) by complexation/release kinetics and solubility isotherm experiments using ultraviolet (UV)-visible spectrophotometry and by the measurement of the dynamics information obtained from T(1) relaxation times and diffusion (DOSY) experiments using nuclear magnetic resonance (NMR) spectroscopy. The complex was prepared at different NFOH and DM-beta-CD molar ratios. The UV-visible measurements were recorded in a spectrophotometer, and NMR experiments were recorded at 20 degrees C on a NMR spectrometer (Varian Inova) operating at 500 MHz. Longitudinal relaxation times were obtained by the conventional inversion-recovery method and the DOSY experiments were carried out using the BPPSTE sequence. The kinetics of complexation revealed that 30 h is enough for stabilization of the NFOH absorbance in presence of cyclodextrin. Solubility isotherm studies show a favorable complexation and increase in solubility when NFOH interacts with cyclodextrin. The analysis of the NMR-derived diffusion coefficients and T(1) relaxation times shows that in the presence of DM-beta-CD, NFOH decreases its mobility in solution, indicating that this antichagasic compound interacts with the cyclodextrin cavity. The release kinetics assays showed that NFOH changes its release profile when in the presence of cyclodextrin due to complexation. This study was focused on the physicochemical characterization of drug-delivery formulations that may serve as potentially new therapeutic options for the treatment of Chagas' disease.

Long acting local anesthetic–polymer formulation to prolong the effect of analgesia

Prolonged postoperative analgesia cannot be achieved using single injections of local anesthetic solutions. The study objective was to evaluate the efficacy and toxicity of a new formulation of bupivacaine loaded in an injectable fatty acid based biodegradable polymer poly(sebacic-co-ricinoleic acid) for producing motor and sensory block when injected near the sciatic nerve. Bupivacaine was dissolved in poly(fatty ester-anhydride) paste and tested for drug release in vitro and in vivo after injection in mice. The efficacy and toxicity of the polymer-drug combination was determined by injecting the polymer formulation near the sciatic nerve of mice and measure the sensory and motor nerve blockade for 48 h, while monitoring the animal general health and the injection site. Seventy percent of the incorporated drug was released during 1 week in vitro. Single injection of 10% bupivacaine in the polymer caused motor and sensory block that lasted 30 h. Microscopic examination of the injection sites revealed only mild infiltration in three of eight examined tissues with no pathological findings for internal organs were found. In conclusion the polymer poly(sebacic-co-ricinoleic acid) is a safe carrier for prolonged activity of bupivacaine.