Review of prolonged local anesthetic action

Development of ropivacaine hydrochloride-loaded dissolving microneedles as a local anesthetic agent: A proof-of-concept

Ropivacaine hydrochloride (RPL) is a local anesthetic agent that has been widely used for the treatment of pain during or after surgery. However, this drug is only available in parenteral dosage form and may contribute to the infiltration of RPL into the plasma, causing some undesirable side effects. Intradermal delivery of RPL using dissolving microneedles may become a promising strategy to deliver such drugs into the skin. This research aimed to develop RPL-loaded dissolving microneedles (DMN-RPLs) as a proof of the concept of intradermal delivery of a local anesthetic. The DMN-RPLs were fabricated using either centrifugation or air-pressurized chamber methods. Several polymers, such as poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA), and sodium hyaluronate (SH), were utilized for manufacturing the DMN-RPLs. The prepared DMN-RPLs were assessed for their thermal properties, chemical bonds, mechanical strength, insertion ability, skin-dissolution study, and drug content. Furthermore, in-skin deposition and dermatokinetic studies were also performed. The results showed that F9 (30 % w/w PVP-4 % w/w SH) and F10 (30 % w/w PVP-5 % w/w PVA) containing 5 % w/w of RPL were the most promising formulations, as shown by their needle height reduction (<10 %) and insertion depth (∼400 μm). Both formulations were also able to deliver more than 60 % of the RPL contained in the DMNs into the epidermis, dermis, and receiver compartment. This study, for the first time, has provided a proof concept to deliver RPL as a local anesthetic using DMNs and the intradermal route, aiming to minimize pain and discomfort during administration and improve the patient's experience.

Prolonged Anesthesia Effects of Locally Administered Ropivacaine via Electrospun Poly(caprolactone) Fibrous Membranes

Prolonged analgesia is important to safeguard the patient's comfort and safety during and after surgery in clinical practice. To meet the demand for prolonged analgesia, medical professionals often resort to increasing drug frequency, which may lead to poor patient compliance and serious complications due to drug overdose. Therefore, it is of great interest to develop controlled-release drug delivery systems for local anesthetics, enabling slow and controlled drug release to prolong the analgesic effect and minimize systemic toxicity. In this study, we utilized an electrospinning technique to fabricate nonwoven poly(caprolactone) (PCL) fibrous membranes loaded with Ropivacaine and performed proof-of-principle experiments on both in vitro drug release tests and in vivo animal tests, to further prolong the analgesic effect of Ropivacaine and improve postoperative local pain management and chronic pain treatment. Material characterization and in vitro drug release studies confirmed the feasibility of the Ropivacaine-loaded PCL fibrous membranes for sustained release. The drug loading content and drug loading efficiency of Ropivacaine-loaded fibrous membrane are 8.7 ± 0.3 wt% and 96 ± 3 wt%, respectively. Evaluation in an animal model demonstrated prolonged anesthesia effects along with excellent biocompatibility and stability. At 72 h, the cumulative release accounted for approximately 50% of the drug loading content. This study offers novel approaches and strategies for clinical postoperative pain management and chronic pain treatment, while providing new insights and directions for the design of local anesthetic controlled-release delivery systems.

Emerging Anesthetic Nanomedicines: Current State and Challenges

Anesthetics, which include both local and general varieties, are a unique class of drugs widely utilized in clinical surgery to alleviate pain and promote relaxation in patients. Although numerous anesthetics and their traditional formulations are available in the market, only a select few exhibit excellent anesthetic properties that meet clinical requirements. The main challenges are the potential toxic and adverse effects of anesthetics, as well as the presence of the blood-brain barrier (BBB), which makes it difficult for most general anesthetics to effectively penetrate to the brain. Loading anesthetics onto nanocarriers as anesthetic nanomedicines might address these challenges and improve anesthesia effectiveness, reduce toxic and adverse effects, while significantly enhance the efficiency of general anesthetics passing through the BBB. Consequently, anesthetic nanomedicines play a crucial role in the field of anesthesia. Despite their significance, research on anesthetic nanomedicines is still in its infancy, especially when compared to other types of nanomedicines in terms of depth and breadth. Although local anesthetic nanomedicines have received considerable attention and essentially meet clinical needs, there are few reported instances of nanomedicines for general anesthetics. Given the extensive usage of anesthetics and the many of them need for improved performance, emerging anesthetic nanomedicines face both unparalleled opportunities and considerable challenges in terms of theory and technology. Thus, a comprehensive summary with systematic analyses of anesthetic nanomedicines is urgently required. This review provides a comprehensive summary of the classification, properties, and research status of anesthetic nanomedicines, along with an exploration of their opportunities and challenges. In addition, future research directions and development prospects are discussed. It is hoped that researchers from diverse disciplines will collaborate to study anesthetic nanomedicines and develop them as a valuable anesthetic dosage form for clinical surgery.

Ferrostatin-1 ameliorates Bupivacaine-Induced spinal neurotoxicity in rats by inhibiting ferroptosis

Bupivacaine (BUP) has previously been shown to trigger neurotoxicity after spinal anesthesia. Further, ferroptosis has been implicated in the pathological processes associated with various central nervous system diseases. Although the impact of ferroptosis on BUP-induced neurotoxicity in the spinal cord has not been fully understood, this research aims to investigate this relationship in rats. Additionally, this study aims to determine whether ferrostatin-1 (Fer-1), a potent inhibitor of ferroptosis, can provide protection against BUP-induced spinal neurotoxicity. The experimental model for BUP-induced spinal neurotoxicity involved the administration of 5% bupivacaine through intrathecal injection. Then, the rats were randomized into the Control, BUP, BUP + Fer-1, and Fer-1 groups. BBB scores, %MPE of TFL, and H&E and Nissl stainings showed that intrathecal Fer-1 administration improved functional recovery, histological outcomes, and neural survival in BUP-treated rats. Moreover, Fer-1 has been found to alleviate the BUP-induced alterations related to ferroptosis, such as mitochondrial shrinkage and disruption of cristae, while also reducing the levels of malondialdehyde (MDA), iron, and 4-hydroxynonenal (4HNE). Fer-1 also inhibits the accumulation of reactive oxygen species (ROS) and restores the normal levels of glutathione peroxidase 4 (GPX4), cystine/glutamate transporter (xCT), and glutathione (GSH). Furthermore, double-immunofluorescence staining revealed that GPX4 is primarily localized in the neurons instead of microglia or astroglia in the spinal cord. In summary, we demonstrated that ferroptosis play a pivotal role in mediating BUP-induced spinal neurotoxicity, and Fer-1 ameliorated BUP-induced spinal neurotoxicity by reversing the underlying ferroptosis-related changes in rats.

Peripheral Regional Anesthesia Using Local Anesthetics: Old Wine in New Bottles?

During the past decade, numerous efforts were undertaken aiming at prolonging the analgesic effect of regional anesthesia. With the development of extended-release formulations and enhanced selectivity for nociceptive sensory neurons, a very promising contribution to the development of pain medications has been achieved. At present, liposomal bupivacaine is the most popular, non-opioid, controlled drug delivery system, but its duration of action, which is still controversially discussed, and its expensiveness have decreased initial enthusiasm. Continuous techniques can be seen as an elegant alternative for providing a prolonged duration of analgesia, but for logistic or anatomical reasons, they are not always the best choice. Therefore, focus has been directed towards the perineural and/or intravenous addition of old and established substances. As for perineural application, most of these so-called 'adjuvants' are used outside their indication, and their pharmacological efficacy is often not or only poorly understood. This review aims to summarize the recent developments for prolonging the duration of regional anesthesia. It will also discuss the potential harmful interactions and side effects of frequently used analgesic mixtures.

Effect of temperature-responsive hydrogel on femoral and sciatic nerve blocks using bupivacaine in Beagle dogs

OBJECTIVES

To compare the duration of regional anesthesia of the pelvic limb using bupivacaine with and without a temperature-responsive hydrogel (TRH) in dogs.

METHODS

Under anesthesia using medetomidine (10 μg·kg^-1 ), alfaxalone (2 mg·kg^-1 ), and isoflurane, seven healthy male Beagles received four injections of 0.5% bupivacaine (1 mg·kg^-1 with 5 μg·ml^-1 epinephrine) to block the femoral and sciatic nerves bilaterally via ultrasound with nerve stimulation guidance. Bupivacaine was used on one pelvic limb (Bup treatment), and bupivacaine with TRH was used on the contralateral limb (Bup-TRH treatment). The nerve block was considered successful upon the absence of responses to pinching the digital pads and mid-tibial skin of both pelvic limbs with mosquito forceps; the pinch, proprioception, and locomotion tests were performed before (baseline) and at each hour after the nerve block until sensory and motor functions returned to baseline. The effect of TRH on nerve blocks was analyzed using a linear mixed model.

RESULTS

The duration of the sensory nerve block at the digital pads and mid-tibial skin was longer with Bup-TRH (8.0 ± 1.6 h and 10.9 ± 1.6 h, respectively) than with Bup treatment (3.7 ± 2.0 h and 8.0 ± 1.6 h, respectively). Motor block times of proprioception and locomotion were longer with Bup-TRH (9.3 ± 1.6 and 12.7 ± 1.5 h, respectively) than with Bup treatment (4.6 ± 1.9 and 9.6 ± 1.5 h, respectively). No complications were observed.

CLINICAL SIGNIFICANCE

TRH extended the duration of regional anesthesia of the pelvic limb using bupivacaine.

Development of local anesthetic drug delivery system by administration of organo-silica nanoformulations under ultrasound stimuli: in vitro and in vivo investigations

The development of local anesthetic (LA) system is the application of commercial drug for the pain management that indorses the reversible obstructive mechanism of neural transmission through preventing the innervation process in human peripheral nerves. Ropivacaine (RV) is one of the greatest frequently used LA s with the actions of long-lasting and low-toxicity for the post-operative pain management. In this work, we have approached novel design and development of glycosylated chitosan (GCS) encapsulated mesoporous silica nanoparticles (GCS-MONPs)-based nano-scaffold for sustainable distributions and controlled/supported arrival of stacked RV for targeting sites, which can be activated by either outer ultrasound activating to discharge the payload, foundation on-request and dependable analgesia. The structural and morphology analyses result established that prepared nano-formulations have successful molecular interactions and RV loaded spherical morphological structures. The drug release profile of developed nanostructure with ultrasound-activation has been achieved 50% of drug release in 2 h and 90% of drug release was achieved in 12 h, which displays more controlled release when compared to free RV solution. The in vitro cell compatibility analysis exhibited GCS-MONPs with RV has improved neuron cell survival rates when compared to other samples due to its porous surface and suitable biopolymer proportions. The analysis of ex vitro and in vivo pain relief analysis demonstrated treated animal models have high compatibility with GCS-MONPs@RV, which was confirmed by histomorphology. This developed MONPs based formulations with ultrasound-irradiation gives a prospective technique to clinical agony the board through on-request and dependable help with discomfort.

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.

Injectable PLGA-Coated Ropivacaine Produces A Long-Lasting Analgesic Effect on Incisional Pain and Neuropathic Pain

The management of persistent postsurgical pain and neuropathic pain remains a challenge in the clinic. Local anesthetics have been widely used as simple and effective treatment for these 2 disorders, but the duration of their analgesic effect is short. We here reported a new poly lactic-co-glycolic acid (PLGA)-coated ropivacaine that was continuously released in vitro for at least 6 days. Perisciatic nerve injection of the PLGA-coated ropivacaine attenuated paw incision-induced mechanical allodynia and heat hyperalgesia during the incisional pain period, and spared nerve injury-induced mechanical and cold allodynia for at least 7 days postinjection. This effect was dose-dependent. Perisciatic nerve injection of the PLGA-coated ropivacaine did not produce detectable inflammation, tissue irritation, or damage in the sciatic nerve and surrounding muscles at the injected site, dorsal root ganglion, spinal cord, or brain cortex, although the scores for grasping reflex were mildly and transiently reduced in the higher dosage-treated groups. PERSPECTIVE: Given that PLGA is an FDA-approved medical material, and that ropivacaine is used currently in clinical practice, the injectable PLGA-coated ropivacaine represents a new and highly promising avenue in the management of postsurgical pain and neuropathic pain.

Near-infrared triggered on-demand local anesthesia using a jammed microgels system

To conveniently modulate the degree of local analgesia in response to changes in patients' needs and level of activity, a NIR-activated drug delivery system based on jammed microgels was introduced in the present study to realize on-demand local anesthesia. Chemically cross-linked gelatin microgels (5-15 μm) containing N-isopropylacrylamide (NIPAM), methylallyl polyethylene glycol (APEG) and graphene oxide (GOs) were fabricated through emulsion. After the in situ free radical polymerization, the physical network was formed, producing microgels with double networks (DN microgels). The DN microgels exhibited thermosensitive properties. The copolymerization of APEG resulted in the increase of lower critical solution temperature (LCST) of microgels. The maximum volume shrinkage ratio of DN microgels (NIPAM40 + APEG60) increased with the increase of the content of physical cross-linking network. The DN microgels also exhibited NIR-responsive ability. Under the NIR irradiance of 272 mW/cm², the temperature of DN microgels with 3 mg/mL GOs reached 40 °C within 60 s, resulting in the volume shrinkage of 14%. Ropivacaine release from DN microgels could be effectively triggered by NIR irradiation in vitro. After centrifugation, a jammed microgels system was produced where microgels packed densely, displaying shear-thinning behavior for achieving injection. The jammed DN microgels carrying ropivacaine were injected subcutaneously into rat footpad. NIR irradiation produced on-demand and repeated infiltration anesthesia in the rat footpad. The jammed DN microgels system thus was beneficial in the management of pain.

Strategies to Obtain Encapsulation and Controlled Release of Small Hydrophilic Molecules

The therapeutic effect of small hydrophilic molecules is limited by the rapid clearance from the systemic circulation or a local site of administration. The unsuitable pharmacokinetics and biodistribution can be improved by encapsulating them in drug delivery systems. However, the high-water solubility, very hydrophilic nature, and low molecular weight make it difficult to encapsulate small hydrophilic molecules in many drug delivery systems. In this mini-review, we highlight three strategies to efficiently encapsulate small hydrophilic molecules and achieve controlled release: physical encapsulation in micro/nanocapsules, physical adsorption via electronic interactions, and covalent conjugation. The principles, advantages, and disadvantages of each strategy are discussed. This review paper could be a guide for scientists, engineers, and medical doctors who want to improve the therapeutic efficacy of small hydrophilic drugs.

Advances of Nano-Structured Extended-Release Local Anesthetics

Extended-release local anesthetics (LAs) have drawn increasing attention with their promising role in improving analgesia and reducing adverse events of LAs. Nano-structured carriers such as liposomes and polymersomes optimally meet the demands of/for extended-release, and have been utilized in drug delivery over decades and showed satisfactory results with extended-release. Based on mature technology of liposomes, EXPAREL, the first approved liposomal LA loaded with bupivacaine, has seen its success in an extended-release form. At the same time, polymersomes has advances over liposomes with complementary profiles, which inspires the emergence of hybrid carriers. This article summarized the recent research successes on nano-structured extended-release LAs, of which liposomal and polymeric are mainstream systems. Furthermore, with continual optimization, drug delivery systems carry properties beyond simple transportation, such as specificity and responsiveness. In the near future, we may achieve targeted delivery and controlled-release properties to satisfy various analgesic requirements.

Recent research and development of local anesthetic-loaded microspheres

This review introduces the recent research and development in local anesthetic-loaded microsphere, as efficient microspheres formulation, the efficient microspheres: optimum preparation method, high loading efficiency, and ideal release rate.

Topical anesthetic analgesic therapy using the combination of ropivacaine and dexmedetomidine: hyaluronic acid modified long-acting nanostructured lipid carriers containing a skin penetration enhancer

Purpose

Hyaluronic acid-poly(ethylene glycol)-distearoyl phosphoethanolamine (HA-PEG-DSPE) modified and tocopheryl polyethylene glycol 1000 succinate (TPGS) contained nanostructured lipid carriers (NLCs) were prepared loading ropivacaine and dexmedetomidine to improve the topical anesthetic analgesic anesthesia efficiency.

Methods

NLCs were prepared by the solvent diffusion method. The average particle size, zeta potential, release behavior, and cytotoxicity of the NLCs were tested. Ex vivo skin permeation was studied using a Franz diffusion cell mounted with depilated rat skin. Local anesthesia antinociceptive efficiency was evaluated by rat tail flick latency study in vivo.

Results

NLCs have sizes of about 100 nm, with negative zeta potentials. All the NLCs formulations were found to be significantly less cytotoxic than free drugs at equivalent concentrations. The cumulative amount of drugs penetrated through rat skin from NLCs was 2.0–4.7 folds higher than that of the drugs solution. The in vivo anesthesia antinociception study displayed that NLCs showed stronger and longer anesthesia antinociceptive effect when compared with single drugs loaded NLCs and drugs solution even at a lower dosage of drugs.

Conclusion

The results demonstrated that the HA modified, TPGS contained, dual drugs loaded NLCs could perform a synergistic effect and may reduce the amount of drugs, which can lower the toxicity of the system and at the meanwhile, increase the anesthesia antinociceptive efficiency.

Sucrose Acetate Isobutyrate as an In situ Forming Implant for Sustained Release of Local Anesthetics

OBJECTIVE

In this study, an injectable Sucrose Acetate Isobutyrate (SAIB) drug delivery system (SADS) was designed and fabricated for the sustained release of Ropivacaine (RP) to prolong the duration of local anesthesia.

METHODS

By mixing SAIB, RP, and N-methyl-2-pyrrolidone, the SADS was prepared in a sol state with low viscosity before injection. After subcutaneous injection, the pre-gel solution underwent gelation in situ to form a drug-released depot.

RESULT

The in vitro release profiles and in vivo pharmacokinetic analysis indicated that RP-SADS had suitable controlled release properties. Particularly, the RP-SADS significantly reduced the initial burst release after subcutaneous injection in rats.

CONCLUSION

In a pharmacodynamic analysis of rats, the duration of nerve blockade was prolonged by over 3-fold for the RP-SADS formulation compared to RP solution. Additionally, RP-SADS showed good biocompatibility in vitro and in vivo. Thus, the SADS-based depot technology is a safe drug delivery strategy for the sustained release of local anesthetics with long-term analgesia effects.

Lipid-based carriers for the delivery of local anesthetics

INTRODUCTION

There is a clinical need for pharmaceutical dosage forms devised to prolong the acting time of local anesthetic (LA) agents or to reduce their toxicity. Encapsulation of LA in drug delivery systems (DDSs) can provide long-term anesthesia for inpatients (e.g. in immediate postsurgical pain control, avoiding the side effects from systemic analgesia) and diminished systemic toxicity for outpatients (in ambulatory/dentistry procedures). The lipid-based formulations described here, such as liposomes, microemulsions, and lipid nanoparticles, have provided several nanotechnological advances and therapeutic alternatives despite some inherent limitations associated with the fabrication processes, costs, and preclinical evaluation models.

AREAS COVERED

A description of the currently promising lipid-based carriers, including liposomes, microemulsions, and nanostructured lipid carriers, followed by a systematic review of the existing lipid-based formulations proposed for LA. Trends in the research of these LA-in-DDS are then exposed, from the point of view of administration route and alternatives for non-traditionally administered LA molecules.

EXPERT OPINION

Considering the current state and potential future developments in the field, we discuss the reasons for why dozens of formulations published every year fail to reach clinical trials; only one lipid-based formulation for the delivery of local anesthetic (Exparel®) has been approved so far.

An efficient and long-acting local anesthetic: ropivacaine-loaded lipid-polymer hybrid nanoparticles for the control of pain

Purpose

Local anesthetics are used clinically for the control of pain following operation (including gastrointestinal surgery) or for the management of other acute and chronic pain. This study aimed to develop a kind of lipid-polymer hybrid nanoparticles (LPNs), which were constructed using poly(ethylene glycol)-distearoylphosphatidylethanolamine (PEG-DSPE) as the hydrophilic lipid shell and poly-ε-caprolactone (PCL) as the hydrophobic polymeric core.

Methods

Ropivacaine (RPV) was entrapped in the LPNs (RPV-LPNs) and the physicochemical and biochemical properties such as size, zeta potential, drug release, and cytotoxicity were studied. The long-lasting effects and safety aspects of the LPNs were evaluated in vitro and in vivo.

Results

The particle size and zeta potential of RPV-LPNs were 112.3±2.6 nm and −33.2±3.2 mV, with an entrapment efficiency (EE) of 90.2%±3.7%. Ex vivo permeation efficiency of LPNs was better than the drug solution. The RPV-LPNs exhibited a long-lasting in vivo anesthesia effect in both rats and mice.

Conclusion

Considering the low cytotoxicity, the LPNs prepared here could be used as an efficient local anesthetic for the control of pain.

Development of bupivacaine decorated reduced graphene oxide and its local anesthetic effect-In vivo study

The present works aims to develop bupivacaine modified reduced graphene oxide (BPV/RGO), and comparative evaluation of their anesthetic effect with free bupivacaine (BPV). The prepared BPV/RGO was studied by using various spectroscopic and microscopic characterization studies. In vitro drug release from BPV/RGO was studied using HPLC analysis. The cytotoxicity of BPV/RGO was studied against fibroblast (3T3) cells. In vivo evaluation of anesthetic effects was performed on animal models. BPV/RGO showed a prolonged in vitro release and lower cytotoxicity when compared to free BPV. Also, BPV/RGO showed a significantly prolonged analgesic effect when compared to free BPV. Further, the prepared BPV/RGO drug delivery system demonstrated to function as gifted to overcome the drawbacks of free BPV and other available drug delivery systems by prolonging the anesthetic effect with poor cytotoxicity.

Injectable in situ forming gel based on lyotropic liquid crystal for persistent postoperative analgesia

Local anesthetics have been widely used for postoperative analgesia. However, multiple injections or local infiltration is required due to the short half-lives of local anesthetics after single injection, which results in poor compliance and increasing medical expense. In this study, an in situ forming gel (ISFG) based on lyotropic liquid crystal was developed to deliver bupivacaine hydrochloride (BUP) for long-acting postoperative analgesia. BUP-ISFG was designed to be administrated as a precursor solution which would spontaneously transform into gel with well-defined internal nanostructures for sustained drug release at the site of administration when exposed to physiological fluid. A lamellar-hexagonal-cubic phase transition occurred during the in situ gelation. The lamellar phase of the precursor solution endows it with low viscosity for good syringeability while the unique nanostructures of hexagonal and cubic phases of the in situ gel provide sustained drug release. Persistent analgesia effect in vivo was achieved with BUP-ISFG, and the plasma BUP concentration was found to be steadier compared to commercially available BUP for injection. In addition, the ISFG displayed acceptable biocompatibility and good biodegradability. The findings are positive about ISFG as a sustained release system for persistent postoperative analgesia.

STATEMENT OF SIGNIFICANCE

To address the issue of insufficient postoperative analgesia associated with short half-lives of local anesthetics after single injection, an in situ forming gel (ISFG) based on lyotropic liquid crystal was developed to deliver bupivacaine hydrochloride (BUP) for postoperative analgesia over three days. The results demonstrated that persistent analgesia effect in vivo was achieved with single injection of BUP-ISFG, and the plasma BUP concentration was found to be steadier compared to commercially available BUP injection. The BUP-ISFG possessed a lamellar-hexagonal-cubic phase transition with corresponding crystal change in 3D nanostructure during the in situ gelation. The relationship between crystal nanostructure and carrier function, might provide some insights to the design and clinical applications of the drug delivery systems based on lyotropic liquid crystal.

Hyaluronic acid modified nanostructured lipid carriers for transdermal bupivacaine delivery: In vitro and in vivo anesthesia evaluation

For effective transdermal local anesthetic therapy, to reduce the barrier of stratum corneum and improve the antinociceptive effect, hyaluronic acid (HA) modified, bupivacaine (BPV) loaded nanostructured lipid carriers (NLCs) were designed. HA and linoleic acid (LOA) conjugated propylene glycol (PEG) was synthesized (HA-PEG-LOA). HA-PEG-LOA was added during the preparation process of NLCs, thus LOA was inserted into the NLCs, The physicochemical properties of NLCs, particle size, zeta potential, drug loading capacity, in vitro skin permeation, drug release profiles and in vivo therapeutic effect were evaluated. HA-BPV/NLCs have small particle size of 150?nm, with a zeta potential of ?40?mV. Nearly 90% high drug encapsulation efficiency and good stability were also observed. In vitro release rate of BPV from HA-BPV/NLCs was complying with a sustained behavior until 72?h of study. HA-BPV/NLCs and BPV/NLCs exhibited 2.5 and 1.6 fold of percutaneous penetration improvement than free BPV. BPV loaded NLCs produced a more prolonged antinociceptive effect when compared with free BPV. In vitro and in vivo results pointed out HA modified NLCs have the capability to act as effective drug carriers, thus prolonging and enhancing the anesthetic effect of BPV. The NLCs developed in this study might provide a useful platform for developing a sophisticated dermal delivery system for analgesic.

Local anaesthetic pain relief therapy: In vitro and in vivo evaluation of a nanotechnological formulation co-loaded with ropivacaine and dexamethasone

Combination therapy is frequently applied to anesthesia and analgesia for its benefits, which includes prolonged analgesia following peripheral nerve blockade, and reduced side effects. The aim of this study was to develop chitosan (CH) coated poly(ε-caprolactone) (PCL) nanoparticles to co-deliver ropivacaine (RPV) and dexamethasone (DEM) (RPV/DEM CH-PCL NPs) for the prolongation of anesthesia and pain relief. In the present study, RPV/DEM CH-PCL NPs were fabricated. The properties of CH-PCL NPs were evaluated for their particle sizes, zeta potential, drug loading capacity and in vitro drug release profile. In vitro skin permeation and in vivo therapeutic effect in an animal model were further investigated. The results showed that the NPs was around 190nm, with PDI of less than 0.20. The zeta potentials of NPs were about 36mV. In vitro drug release of both RPV and DEM from NPs complied with sustained behaviors. All of the drugs loaded NPs samples studied exhibited no obvious L929 cells cytotoxicity. In vitro skin penetration profiles showed the amount of RPV permeated through the skin from NPs was significantly higher than free RPV. RPV and DEM co-loaded NPs induced remarkably better anesthetic effect than non DEM loaded RPV CH-PCL NPs. The results suggested that adding a small dosage of DEM could improve the anesthesia efficacy of RVP to a large content. The resulting formulation could be applied as a promising anesthesia system for local anesthetics therapy.

Encapsulation of ropivacaine in a combined (donor-acceptor, ionic-gradient) liposomal system promotes extended anesthesia time

Ropivacaine is a local anesthetic with similar potency but lower systemic toxicity than bupivacaine, the most commonly used spinal anesthetic. The present study concerns the development of a combined drug delivery system for ropivacaine, comprised of two types of liposomes: donor multivesicular vesicles containing 250 mM (NH₄)₂SO₄ plus the anesthetic, and acceptor large unilamellar vesicles with internal pH of 5.5. Both kinds of liposomes were composed of hydrogenated soy-phosphatidylcholine:cholesterol (2:1 mol%) and were prepared at pH 7.4. Dynamic light scattering, transmission electron microscopy and electron paramagnetic resonance techniques were used to characterize the average particle size, polydispersity, zeta potential, morphology and fluidity of the liposomes. In vitro dialysis experiments showed that the combined liposomal system provided significantly longer (72 h) release of ropivacaine, compared to conventional liposomes (~45 h), or plain ropivacaine (~4 h) (p <0.05). The pre-formulations tested were significantly less toxic to 3T3 cells, with toxicity increasing in the order: combined system < ropivacaine in donor or acceptor liposomes < ropivacaine in conventional liposomes < plain ropivacaine. The combined formulation, containing 2% ropivacaine, increased the anesthesia duration up to 9 h after subcutaneous infiltration in mice. In conclusion, a promising drug delivery system for ropivacaine was described, which can be loaded with large amounts of the anesthetic (2%), with reduced in vitro cytotoxicity and extended anesthesia time.

Local anesthetic effects of bupivacaine loaded lipid-polymer hybrid nanoparticles: In vitro and in vivo evaluation

PURPOSE

There is a compelling need for prolonged local anesthetic that would be used for analgesia with a single administration. However, due to the low molecular weight of local anesthetics (LA) (lidocaine, bupivacaine, procaine, dibucaine, etc), they present fast systemic absorption.

METHODS

The aim of the present study was to develop and evaluate bupivacaine lipid-polymer hybrid nanoparticles (BVC LPNs), and compared with BVC loaded PLGA nanoparticles (BVC NPs). Their morphology, particle size, zeta potential and drug loading capacity were evaluated. In vitro release study, stability and cytotoxicity were studied. In vivo evaluation of anesthetic effects was performed on animal models.

RESULTS

A facile nanoprecipitation and self-assembly method was optimized to obtain BVC LPNs, composed of PLGA, lecithin and DSPE-PEG₂₀₀₀, of ∼175nm particle size. Compared to BVC NPs, BVC LPNs exhibited prolonged in vitro release in phosphate-buffered saline (pH=7.4). Further, BVC LPNs displayed enhanced in vitro stability in 10% FBS and lower cytotoxicity (the concentration of BVC ranging from 1.0μM to 20μM). In addition, BVC LPNs exhibited significantly prolonged analgesic duration.

CONCLUSION

These results demonstrate that the LPNs could function as promising drug delivery system for overcoming the drawbacks of poor stability and rapid drug leakage, and prolonging the anesthetic effect with slight toxicity.

Drug delivery systems for prolonged duration local anesthesia

Numerous drug delivery systems have been applied to the problem of providing prolonged duration local anesthesia (PDLA). Here we review the rationale for PDLA, the desirable features for and important attributes of such systems, and specific examples that have been developed.

New Updates Pertaining to Drug Delivery of Local Anesthetics in Particular Bupivacaine Using Lipid Nanoparticles

Lipid nanoparticles (liposomes) were first described in 1965, and several work have led to development of important technical advances like triggered release liposomes and drug-loaded liposomes. These advances have led to numerous clinical trials in such diverse areas such as the delivery of anti-cancer, antifungal, and antibiotic drugs; the delivery of gene medicines; and most importantly the delivery of anesthesia drugs. Quite a number of liposomes are on the market, and many more are still in developmental stage. Lipid nanoparticles are the first nano-medicine delivery system to be advanced from laboratory concept to clinical application with high considerable clinical acceptance. Drug delivery systems for local anesthetics (LAs) have caught the interest of many researchers because there are many biomedical advantages connected to their application. There have been several formulation techniques to systemically deliver LA that include encapsulation in liposomes and complexation in cyclodextrins, nanoparticles, and to a little extent gold nanoparticles. The proposed formulations help to decrease the LA concentration utilized, increase its permeability, and most importantly increase the localization of the LA for a long period of time thereby leading to increase in the duration of the LA effect and finally to reduce any local and systemic toxicity. In this review, we will highlight on new updates pertaining to drug delivery of local anesthetics in particular bupivacaine using lipid nanoparticles.

A Novel Proliposomal Ropivacaine Oil: Pharmacokinetic-Pharmacodynamic Studies After Subcutaneous Administration in Pigs

BACKGROUND

Liposomal local anesthetics are limited by a short liposomal shelf-life, even when under refrigeration. We describe a novel proliposomal ropivacaine that produces liposomes in situ, only after exposure to aqueous media.

METHODS

In vitro: Nanoparticles were assessed (particle size distribution analyzer, cryo-transmission electron microscopy) at baseline and after exposure to saline/plasma.

TOXICITY

In porcine wound healing study (n = 12), healing was assessed by photography, clinical assessment, and histology. Pharmacodynamics: Seventeen young piglets were randomly assigned to plain 0.5% ropivacaine (n = 5), proliposomal 4% ropivacaine (n = 6), or sham (n = 6). Tactile threshold was assessed using von Frey filaments applied to the surgical wound; the nonoperated skin was used as a control. Tactile threshold over time was determined using area under the curve (AUC) and assessed by 1-way analysis of variance.

PHARMACOKINETICS

8 young piglets were randomly assigned to plain 0.5% (25 mg, n = 4) or proliposomal 4% (200 mg, n = 4) ropivacaine. Plasma ropivacaine was assessed by high-performance liquid chromatography at baseline and at intervals over 36 hours. Paired ropivacaine concentration (from wound exudate and plasma) was obtained at 96 hours. Data were analyzed using noncompartmental and compartmental models.

RESULTS

In vitro: On exposure to saline and plasma, the study drug was transformed from a homogenous oil to an emulsion containing liposomes of approximately 1.4-μm diameter; this effect was dilution dependent and stable over time.

TOXICITY

All wounds healed well; no effect of drug group was observed. Pharmacodynamics: Plain and proliposomal ropivacaine provided sensory anesthesia for approximately 6 and 30 hours, respectively. There was an approximately 7-fold increase in the AUC of anesthesia for proliposomal ropivacaine compared with plain ropivacaine (mean difference, 1010; 95% confidence interval [CI], 625-1396 g·h/mm; P < 0.0001).

PHARMACOKINETICS

There was no difference in Cmax (2.31 ± 0.74 vs 2.32 ± 0.46 mg/L), despite an approximately 8-fold difference in dose. However, proliposomal ropivacaine was associated with a marked prolongation of Tmax (6.50 ± 6.35 vs 0.5 ± 0.0 hours), terminal half-life (16.07 ± 5.38 vs 3.46 ± 0.88 hours; P = 0.0036), and ropivacaine-time AUC (47.72 ± 7.16 vs 6.36 ± 2.07 h·mg/L; P < 0.0001), when compared with plain ropivacaine. The proliposomal formulation provided an approximately 250-fold higher ropivacaine concentration in the surgical wound (mean difference, 3783 ng/mL; 95% CI, 1708-5858; P = 0.001) and an approximately 25-fold higher wound:plasma ropivacaine concentration ratio (mean difference, 126; 95% CI 38-213; P = 0.011).

CONCLUSIONS

Proliposomal ropivacaine exerted prolonged anesthesia with delayed elimination, typical for liposomal drugs. The advantage of this novel proliposomal ropivacaine is its ease of preparation and its extended shelf-stability (>2 years) at room temperature.

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.

Transcriptional transactivator peptide modified lidocaine-loaded nanoparticulate drug delivery system for topical anesthetic therapy

PURPOSE

For the topical anesthetic, transcriptional transactivator peptide (TAT) modified lidocaine (LID) loaded nanostructured lipid carriers (TAT-NLCs-LID) were prepared and then used for improving transdermal delivery of local anesthetic drug.

METHODS

In this study, TAT was conjugated with Distearoyl phosphatidylethanolamine-(polyethylene glycol)₂₀₀₀-maleimide (DSPE-PEG₂₀₀₀-Mal) to obtain TAT-PEG₂₀₀₀-DSPE. TAT-NLCs-LID were successfully prepared and characterized by determination of their particle size, morphology, drug encapsulation efficiency and in vitro drug release behavior. The skin permeation of LID-LNPs was examined using a Franz diffusion cell mounted with depilated mouse skin in vitro and in vivo anesthesia effect was evaluated on mice.

RESULTS

The results showed that TAT-NLCs-LID have substantially small mean diameter (157.9 nm) and high encapsulation efficiency (81.8%). From the in vitro skin permeation results, transdermal flux of TAT-NLCs-LID was about several times higher than that of LID solution and NLCs-LID. In vivo anesthesia effect evaluation illustrated that TAT-NLCs-LID can enhance the transdermal delivery of LID by reducing the pain threshold in mice.

CONCLUSION

These results indicate that the novel TAT containing drug delivery system is very useful for overcoming the barrier function of the skin and could deliver anesthetic through the skin. TAT-NLCs-LID could function as promising topical anesthetic system.

Preparation and investigation of a novel levobupivacaine in situ implant gel for prolonged local anesthetics

In this study, levobupivacaine (LBP) was successfully incorporated into implant application based on the concept of in situ gel (PECE). The physicochemical characterization (preparation, phase transition temperature, in vitro release) were investigated. The results of viscosity measurement showed that the preparations behaved like a fluid but formed a rigid gel when exposed to increasing temperature (37 °C). In vitro release, LBP was graduated released from the gels as time lapsed, suggesting that LBP was well entrapped in PECE in situ gels. As is shown in pharmacokinetic parameters, the half-life of LBP injection (2.7 h) was shorter than that of LBP in situ gels (23.9 h), suggesting that LBP injection was taken up by other tissues more rapidly than gels. The area under the curve of LBP in situ gels was 2.18 times the size of that of LBP injection (P< 0.05). In pharmacodynamic test, even after nine hours of injection, gel group could still maintain rather good anesthesia effect and rats' stinging reaction maintained at a relatively low level, which had obvious statistical differences compared to injection group.

Physiological and pharmacologic aspects of peripheral nerve blocks

A successful peripheral nerve block not only involves a proper technique, but also a thorough knowledge and understanding of the physiology of nerve conduction and pharmacology of local anesthetics (LAs). This article focuses on what happens after the block. Pharmacodynamics of LAs, underlying mechanisms of clinically observable phenomena such as differential blockade, tachyphylaxis, C fiber resistance, tonic and phasic blockade and effect of volume and concentration of LAs. Judicious use of additives along with LAs in peripheral nerve blocks can prolong analgesia. An entirely new group of drugs-neurotoxins has shown potential as local anesthetics. Various methods are available now to prolong the duration of peripheral nerve blocks.

Novel polymeric bioerodable microparticles for prolonged-release intrathecal delivery of analgesic agents for relief of intractable cancer-related pain

Intractable cancer-related pain complicated by a neuropathic component due to nerve impingement is poorly alleviated even by escalating doses of a strong opioid analgesic. To address this unmet medical need, we developed sustained-release, bioerodable, hydromorphone (potent strong opioid)- and ketamine (analgesic adjuvant)-loaded microparticles for intrathecal (i.t.) coadministration. Drug-loaded poly(lactic-co-glycolic acid) (PLGA) microparticles were prepared using a water-in-oil-in-water method with evaporation. Encapsulation efficiency of hydromorphone and ketamine in PLGA (50:50) microparticles was 26% and 56%, respectively. Microparticles had the desired size range (20-60 μm) and in vitro release was prolonged at ≥28 days. Microparticles were stable for ≥6 months when stored refrigerated protected from light in a desiccator. Desirably, i.t. injected fluorescent dye-labeled PLGA microparticles in rats remained in the lumbar region for ≥7 days. In a rat model of neuropathic pain, i.t. coinjection of hydromorphone- and ketamine-loaded microparticles (each 1 mg) produced analgesia for 8 h only. Possible explanations include inadequate release of ketamine and/or hydromorphone into the spinal fluid, and/or insufficient ketamine loading to prevent development of analgesic tolerance to the released hydromorphone. As sub-analgesic doses of i.t. ketamine at 24-48 h intervals restored analgesia on each occasion, insufficient ketamine loading appears problematic. We will investigate these issues in future work.

Emulsified isoflurane enhances thermal transient receptor potential vanilloid-1 channel activation-mediated sensory/nociceptive blockade by QX-314

BACKGROUND

QX-314 produces nociceptive blockade, facilitated by permeation through transient receptor potential vanilloid-1 (TRPV1) channels. TRPV1 channel can be activated by noxious heat and sensitized by volatile anesthetics. The authors hypothesized that emulsified isoflurane (EI) could enhance thermal TRPV1 channel activation-mediated sensory/nociceptive blockade by QX-314.

METHODS

Rats were perineurally injected with QX-314 (Sigma-Aldrich Co. Ltd. Shanghai, China) alone or QX-314 combined with EI, followed by heat exposure on the injection site. The tail-flick and tail-clamping tests were used to assess sensory and nociceptive blockade, respectively; a sciatic nerve block model was used to assess motor and sensory blockade. Effects of EI on thermal activation of TRPV1 channels were evaluated on rat dorsal root ganglia neurons by whole-cell patch-clamp recordings.

RESULTS

Heat exposure enhanced sensory/nociceptive blockade by QX-314 in rat tails, but not motor blockade in sciatic nerve block model. QX-314 alone or QX-314 + 42°C produced no nociceptive blockade. QX-314 + 48°C produced 100% nociceptive blockade with duration of 12.5 ± 2.0 h (mean ± SEM). By adding 2% EI, QX-314 + 42°C produced 80% nociceptive blockade with duration of 8.1 ± 1.9 h, which was similar to the effect of QX-314 + 46°C (7.7 ± 1.1 h; P = 0.781). The enhancement of heat on sensory/nociceptive blockade of QX-314 was prevented by TRPV1 channel antagonist. The temperature thresholds of TRPV1 channel activation on dorsal root ganglia neurons were significantly reduced by EI.

CONCLUSIONS

Thermal activation of TRPV1 channels enhanced long-lasting sensory/nociceptive blockade by QX-314 without affecting motor blockade. The addition of EI reduced temperature thresholds for inducing long-lasting sensory/nociceptive blockade due to QX-314.

The role of continuous peripheral nerve blocks

A continuous peripheral nerve block (cPNB) is provided in the hospital and ambulatory setting. The most common use of CPNBs is in the peri- and postoperative period but different indications have been described like the treatment of chronic pain such as cancer-induced pain, complex regional pain syndrome or phantom limb pain. The documented benefits strongly depend on the analgesia quality and include decreasing baseline/dynamic pain, reducing additional analgesic requirements, decrease of postoperative joint inflammation and inflammatory markers, sleep disturbances and opioid-related side effects, increase of patient satisfaction and ambulation/functioning improvement, an accelerated resumption of passive joint range-of-motion, reducing time until discharge readiness, decrease in blood loss/blood transfusions, potential reduction of the incidence of postsurgical chronic pain and reduction of costs. Evidence deriving from randomized controlled trials suggests that in some situations there are also prolonged benefits of regional anesthesia after catheter removal in addition to the immediate postoperative effects. Unfortunately, there are only few data demonstrating benefits after catheter removal and the evidence of medium- or long-term improvements in health-related quality of life measures is still lacking. This review will give an overview of the advantages and adverse effects of cPNBs.

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.

Characterization of bupivacaine-loaded formulations based on liquid crystalline phases and microemulsions: the effect of lipid composition

This report details the structural characterization and the in vitro drug-release properties of different local anesthetic bupivacaine (BUP)-loaded inverted-type liquid crystalline phases and microemulsions. The effects of variations in the lipid composition and/or BUP concentration on the self-assembled nanostructures were investigated in the presence of the commercial distilled glycerol monooleate Myverol 18-99K (GMO) and medium-chain triglycerides (MCT). Synchrotron small-angle X-ray scattering (SAXS) and rotating dialysis cell model were used to characterize the BUP formulations and to investigate the in vitro BUP release profiles, respectively. The evaluation of SAXS data for the BUP-loaded GMO/MCT formulations indicates the structural transition of inverted-type bicontinuous cubic phase of the symmetry Pn3m → inverted-type hexagonal (H(2)) phase → inverted-type microemulsion (L(2)) with increasing MCT content (0-40 wt %). In the absence of MCT, the solubilization of BUP induces the transition of Pn3m → H(2) at pH 7.4; whereas a transition of Pn3m → (Pn3m + H(2)) is detected as the hydration is achieved at pH 6.0. To mimic the drug release and transport from in situ formed self-assembled systems after subcutaneous administration, the release experiments were performed by injecting low viscous stimulus-responsive precursors to a buffer in the dialysis cell leaving the surface area between the self-assembled system and the release medium variable. Our results suggest that the pH-dependent variations in the lipidic partition coefficient, K(l/w), between the liquid crystalline nanostructures and the surrounding buffer solution are significantly affecting BUP release rates. Thus, a first step toward understanding of the drug-release mechanism of this drug-delivery class has been undertaken tackling the influence of drug ionization as well as the type of the self-assembled nanostructure and its release kinetics under pharmaceutically relevant conditions.

New delivery systems for local anaesthetics-part 2

Part 2 of this paper deals with the techniques for drug delivery of topical and injectable local anaesthetics. The various routes of local anaesthetic delivery (epidural, peripheral, wound catheters, intra-nasal, intra-vesical, intra-articular, intra-osseous) are explored. To enhance transdermal local anaesthetic permeation, additional methods to the use of an eutectic mixture of local anaesthetics and the use of controlled heat can be used. These methods include iontophoresis, electroporation, sonophoresis, and magnetophoresis. The potential clinical uses of topical local anaesthetics are elucidated. Iontophoresis, the active transportation of a drug into the skin using a constant low-voltage direct current is discussed. It is desirable to prolong local anaesthetic blockade by extending its sensory component only. The optimal release and safety of the encapsulated local anaesthetic agents still need to be determined. The use of different delivery systems should provide the clinician with both an extended range and choice in the degree of prolongation of action of each agent.

New formulations of local anaesthetics-part I

Part 1 comments on the types of local anaesthetics (LAs); it provides a better understanding of the mechanisms of action of LAs, and their pharmacokinetics and toxicity. It reviews the newer LAs such as levobupivacaine, ropivacaine, and articaine, and examines the newer structurally different LAs. The addition of adjuvants such as adrenaline, bicarbonate, clonidine, and corticosteroids is explored. Comment is made on the delivery of topical LAs via bioadhesive plasters and gels and controlled-release local anaesthetic matrices. Encapulation matrices such as liposomes, microemulsions, microspheres and nanospheres, hydrogels and liquid polymers are discussed as well. New innovations pertaining to LA formulations have indeed led to prolonged action and to novel delivery approaches.

Treatment of acute postoperative pain

Although postoperative pain remains incompletely controlled in some settings, increased understanding of its mechanisms and the development of several therapeutic approaches have substantially improved pain control in past years. Advances in our understanding of the process of nociception have led to insight into gene-based pain therapy, the development of acute opioid-induced hyperalgesia, and persistent postsurgical pain. Use of specific analgesic techniques such as regional analgesia could improve patient outcomes. We also examine the development of new analgesic agents and treatment modalities and regimens for acute postoperative pain.