Development and Evaluation of Nanoemulsion Loaded Metaxalone for the Treatment of Pain and Injury

: Spasticity is a disorder characterized by an unnatural rise in muscular tone or sti ﬀ ness, which may impair speech or movement and be accompanied by discomfort or pain. Spasticity can occur due to upper motor neuron dysfunction, which comes about when there is disturbance of inhibitory descending spinal motor pathways. The aim of the present work is formulated nanoemulsion loaded metaxalone and evaluate it for various parameters. Metaxalone is used with rest, physical therapy, and other measures to relax muscles and relieve pain and discomfort caused by strains, sprains, and other muscle injuries. The phase titration method was use to plot pseudo ternary phase diagram to select the ratio of oil and surfactant. The nanoemulsion was prepared by the high-speed homogenization method and an in-vitro drug release study was conducted using a Franz-di ﬀ usion cell. The optimized batch showed the highest entrapment e ﬃ ciency, up to 93%, and the zeta potential − 33 mV and PDI 0.321 showed stable and homogenous behavior of the globule formed. It also showed in-vitro release of up to 8 h following zero order release. Therefore, we conclude that nanoemulsion containing metaxalone showed a prolonged e ﬀ ect compared to plain metaxalone and can e ﬀ ectively work to improve the muscle conditions to relive pain and injury.


Introduction
Spasticity is a clinical condition of upper motor neuron dysfunction, which comes about when there is disturbance of inhibitory descending spinal motor pathways.It is portrayed by a velocity dependent increment in tonic stretch reflexes (muscle tone) with heightened tendon jerks, caused due to hyper excitability of the stretch reflex.It is assessed that the pervasiveness of spasticity worldwide is more than 12 million patients.The reason behind such surge in velocity-dependent increase in tonic stretch reflexes, are the neurological disorders such as cerebral palsy (CP), spinal cord injury (SCI) and stroke [1].Cerebral palsy is the most common cause of physical disability in children, with a reported incidence of 2 to 2.5 per 1000 live births.Approximately 90% of affected children present with clinical symptoms of spastic paresis, a muscle-tone and muscle control-regulation disorder [2].Spasticity is a common symptom after stroke, arising in about 30% of patients, and usually occurs within the first few days or weeks [3].The spinal cord carries all messages between the brain and the rest of the body that involve voluntary motion, sensory perception, and muscle tone.Prevalence of SCI is 250,000 to 300,000 plus 10,000 new injuries per year [4].
Nanoemulsions are transparent or translucent system of nanometric size (10-500 nm) consisting of a dispersed phase, a continuous phase, which is stabilized by a surfactant & cosurfactant [5].Unlike micro emulsions which are thermodynamically stable, nanoemulsions are stable kinetically.This long-term physical stability property of nanoemulsions without any flocculation or coalescence makes them exclusive systems.A nanoemulsion is a biphasic system i.e., it has both hydrophilic and lipophilic heads and it is thermodynamically unstable and stabilized with the addition of emulsifying agent.Nanoemulsions are made up of oils, emulsifying agents (surfactants/co-surfactants) and aqueous phase [6][7][8].
Metaxalone, a muscle relaxant, is used with rest, physical therapy, and other measures to relax muscles and relieve pain and discomfort caused by strains, sprains, and other muscle injuries.Metaxalone comes as a tablet to take by mouth.It usually is taken three or four times a day [9].Side effects may include drowsiness, dizziness, nausea, vomiting, and upset stomach [10].Metaxalone has an onset of action of 1 h, a plasma half-life of 2 to 3 h, and a duration of action of 4 to 6 h.This drug is supplied as 400-mg tablets and has a recommended dose of 800 mg 3 or 4 times daily [11].
The aim of present study was to formulate and evaluate metaxalone loaded nanoemulsion suitable for the oral delivery which would be more effective as skeletal muscle relaxant for the treatment of pain and injury.

Materials and Methods
Metaxalone was received as a gift sample from Micro Labs Pvt. Ltd., Mumbai, Maharashtra, India.Sesame oil, Tween 80, PEG400, Dichloromethane, Triethanolamine, Methyl Paraben, Propyl Paraben were purchased from Research-Lab Fine Chem Industries, Mumbai, Maharashtra, India.

Preparation of Metaxalone Loaded Nanoemulsion
Optimized metaxalone nanoemulsion was prepared by high-speed stirring, followed by probe sonication.Two phases, namely oil phase (P1) and aqueous phase (P2) were compounded separately.In P1, 400 mg metaxalone was dissolved in 5 mL methanol and this was added to oil (10 mL) with continuous stirring for 3 h at 50 °C for complete removal of solvent, and then the surfactant was added to it.For P2, the co-surfactant was solubilized in distilled water (20 mL) and stirring was done for 30 min (Table 1).Then, P1 was gradually added into P2 with continued high-speed stirring (21500 rpm) by Ultraturrax T25 (IKA WERKE, Bengaluru, India) to form a coarse emulsion (Table 1).The formed coarse emulsion was then probe sonicated to obtain fine nano-sized emulsion (Figure 1).The prepared nanoemulsion batches were optimised using metrics such as entrapment efficiency, TEM analysis, Polydispersibility index and Zeta potential.Optimized batch was used for further characterization.

Results and Discussion
UV spectrophotometry was used for determination of λmax and plotting of calibration curve of drug in methanol and in 0.1 N HCL for the confirmation of drug.The compatibility between the drug and the excipients was confirmed using FTIR method.

% Entrapment Efficiency
The highest entrapment efficiency of 93.85 ±1.19% was shown by formulation NE5 as compare to other formulations.

Transmission Electron Microscopy (TEM)
TEM image (Figure 2) of globule shows particle size 300-500 nm which is might be acceptable for delivery nanoemulsion through oral route.

Zeta Potential
The zeta potential of the NE5 formulation was calculated using zeta-sizer.The zeta potential was determined to be −33.3 ± 8.29 mV, which indicate the globules were stable in the prepared formulation (Figure 3a).The average size of globules was found to be 326 ± 91.21 nm and polydispersity index (PDI) was found to be 0.326 that estimates the homogeneity of the particles within the formulation (Figure 3b).

Determination of pH
The pH of nanoemulsion was found to be in range of 1.5-3.5.This range of pH is acceptable for Gastrointestinal tract.

Viscosity
The viscosity of the nanoemulsion was evaluated using a Brookfield viscometer (DV2T model) with a LV-62 cylindrical.NE5 had a suitable viscosity when compared to other formulations.

% Drug Content
Formulation NE5 has shown the highest drug content of 95.67± 3.81%.The drug content was found to be in range of 88.62 ± 4.51% to 95.67 ± 3.81%.

In Vitro Release and Kinetic Modelling
The NE5 formulation showed the maximum drug release upto 94.54 ± 5.82% at 8 hr.The kinetic studies states that the NE5 formulation follows zero order model.

Stability Studies
The optimized nanoemulsion formulation nanoemulsion was stored at 40 ± 2 °C/75% RH in a stability chamber for 90 days.Sample was withdrawn periodically and evaluated for pH, % drug content and in-vitro drug diffusion was found to be optimum and satisfactory and there was no significant change in the formulation.

Conclusions
Metaxalone-loaded nanoemulsion was prepared by combination of homogenization and sonication technique appears to be effective for the preparation of drug loaded nanoemulsion for oral drug delivery.Characterization of Metaxalone-loaded nanoemulsion demonstrated the highest entrapment efficiency up to 93.92 ± 1.19%.The nanoemulsions' polydispersity index (PDI) was 0.321 and their zeta potential was −33.33 ± 4.84 mV, showing that the formulation was stable.The nanoemulsions drug content was found to be 95.67 ± 3.81%.The best results were obtained with a nanoemulsion made with 70%