Formulation and Evaluation of Sustained Release Ondansetron Poloxamer Based Solid Suppositories

The present investigation studies the effect of water swellable polymer hydroxylpropylmethyl cellulose (HPMC K4M, Methocel) on in vitro release of ondansetron from suppositories. Suppositories were prepared by using mixture of Poloxamer 407 and Poloxamer 188 hydrophilic bases. Suppositories containing 16 mg of ondansetron were prepared by fusion method. Weight variation, content uniformity, breaking (hardness), disintegration time, melting point and liquefaction time of the formulations were determined. In vitro release test was carried out according to USP XXII basket method. In vitro release data demonstrates ondansetron release from suppositories up to 12hrs and follows the zero order kinetics from poloxamer mixture based suppositories.


INTRODUCTION
Conventional suppository are made up of polyethylene glycol (PEG) base, which may softens or melts after considerable time in the rectum due to its relatively high melting point and thus cannot be rapidly adsorbed in rectal mucous membranes (Burstein et al., 2000).Furthermore, such PEG based suppository, which may reach the end of the colon due to their higher melting point and results into the loss of drug due to first-pass effect (Kim et al., 1998).To solve these problems of conventional suppository, it would be desirable to develop a novel solid suppository, which is solid in phase at room temperature but instantly melts at physiological temperature and retains for longer duration in rectum instead of colon.Such a suppository must have the suppository base with the suitable melting point (30-36ºC) & mucoadhesive property.
In this study, novel poloxamer-based suppository have been developed with a mixture of poloxamer 407 & poloxamer 188 with melting point of about 15 & 55º C respectively (Abhaham C., 1994).In addition to lower melting point of P124 & P188, they are known to have suitable mucoadhesive force (Choi et al., 1998), low toxicity (Yun et al., 1999), less skin irritation (Choi et al., 1998), compatibility with other chemicals and good drug release characteristics.According to biopharmaceutical classification system, Ondansetron comes under class II with low solubility and high permeability.Ondansetron is chemically 1,2,3,9-tetrahydro -9-methyl-3-[(2-methyl-1-H-imidazole-1-y1) methyl]-4H carbazol -4-one which is highly selective 5-HT3 receptor antagonist and numerous studies have demonstrated its superior anti-emetic efficacy over high dose metoclopramide in the prevention of nausea and vomiting (Fumoleau et al., 1997).Ondansetron is currently available as a tablet (Zofran tablet), an intravenous formulation (Zofran injection).The intravenous form is not suitable for patients as it requires the ambulatory setting and has to be administered by nurse or doctor.The tablet form is not appropriate in patients with chemotherapy induced nausea and vomiting.Thus the aim of the present study is to develop a novel poloxamer-based mucoadhesive solid suppository system of ondansetron which will sustain the release of ondansetron, melts at physiological temperature and has prolonged retention time by adhering to rectal mucosa in patients with chemotherapy induced nausea and vomiting.

Preparation of suppositories
Conventional and sustained release suppositories were prepared by fusion method.Calculated amount of poloxamer 188 and poloxamer 407 were melted individually at 40 0 C and mixed, ondansetron was dispersed homogeneously into melted base.Finally, these mixtures were poured into moulds (1 g capacity) and the moulds were left in ice bath at 4 0 C. The obtained suppositories were sealed in aluminum packaging coated inside with polyethylene and stored at 10 0 C. (Composition shown in Table I.).Sustained release suppositories were prepared using same method used for preparation of conventional suppository with addition of HPMC in different concentration as shown in composition Table I.

DSC Study
The possibility of any interaction between ondansetron and excipients used in formulations was assessed by carrying out the thermal analysis on pure drug and formulation using differential scanning calorimeter.The thermograms of samples were obtained at a scanning rate of 10 0 C / min conducted over range of 50-300 0 C.

Uniformity of weight
Twenty suppositories were selected randomly from each batch and weighed individually and also the average weight and the percentage deviation values were calculated (babar et al., 1999).

Hardness (breaking) test
The hardness of three suppositories from each batch was determined by cutting the middle portion of suppository.It was measured in its diametric direction using Monsanto hardness tester.

Melting point
Melting point of suppository was measured by capillary method using melting point apparatus (SCIENTIFIC).

Disintegration test
Randomly six suppositories were selected from each batch for disintegration test.Disintegration test was performed without disc in phosphate buffer pH 7.2 using USP disintegration apparatus (Electrolab ED-2L) (Stanislav et al., 2001).

Content uniformity
Content uniformity of suppositories was determined by spectrophotometric method.The suppositories were individually melted and dissolved in 100 ml phosphate buffer.

Liquefaction time
It measures the time necessary for suppository to liquefy under pressure similar to those found in rectum in presence of water at body temperature.Liquefaction time measured by using modified apparatus (Fig. 3).
A condenser was taken having broad opening at one end and narrow opening at another end.Dialysis membrane tubing passed through condenser tied at both ends of condenser.The sample suppository was introduced from the top of condenser through broad end and carefully pushed up to some extent.Water (at 37 0 C) was circulated through condenser.When temperature of dialysis membrane tubing was stabilized at 37 0 C, suppository moved downwards and dropped into beaker.The time required for suppository to move from broad end to narrow end was measured (Larry et al., 1991).

In vitro Release Test
In vitro release test was carried out according to the USP XXII basket method.The USP rotating basket dissolution apparatus was used for the determination of release rates of Ondansetron from the various suppository bases.Each suppository was placed in basket and lowered into a flask containing 900 ml of phosphate buffer solution (pH 7.2) with 0.5% sodium lauryl sulphate.The basket was rotated at 50 rpm at a constant temperature 37 0 C ± 0.5 0 C. Aliquots of 5 ml were withdrawn at appropriate time intervals and immediately replaced by 5 ml fresh phosphate buffer.Drug concentration was measured spectrophotometrically at 267 nm.(Shimadzu, UV-1700, Japan).This test was performed on 6 suppositories and mean ± SD was calculated.

RESULTS AND DISCUSSION
During the manufacturing of suppositories, some difficulties are experienced in achieving the exact dosage.This is because the volume of suppositories from particular mold is uniform, but its weight may vary because the density of drug usually differs from density of base with which the mold is calibrated.Therefore, displacement values of Ondansetron from bases were determined and were found to be 0.02 for poloxamer mixture.

Physicochemical characteristics
The physicochemical characteristics of suppositories were measured according to the methods described.The results are listed in Table II.The results show that all prepared suppositories were acceptable in terms of weight variation, content uniformity, hardness, disintegration time, melting point and liquefaction time.Weight variation is in conformity with BP (British pharmacopoeia) with standard deviation less than 5% 17 .The drug content was within the limit of 75% to 125%.The hardness of conventional suppository is less than sustained release suppositories, this may be due to increase in concentration of HPMC.According to BP the disintegration time of each suppository should be less than 60 min for hydrophilic suppository which was found to be within limit.The appropriate melting point of a suppository ensures its handling and release of drug after administration in the rectum (Yahagi et al., 1999).Poloxamer 407 and poloxamer 188 have melting points 15 0 C and 55 0 C respectively but in case of mixture melting point was found to be 32 -33.5 0 C (Chul Soon et al., 2005).

In vitro release test
In vitro release study was carried out using USP XXII basket method, the drug was released directly to the aqueous medium from the surface of suppository.The result shows all conventional suppositories release the drug within 90 minutes.The Formulations P2, P3 and P4 suppositories containing poloxamer mixture base with HPMC showed sustained release up to 12 hours, this is because HPMC has an inert matrix structure and hence forms cage on the surface and the suppositories, thus preventing release of drug from the suppository to the aqueous medium (Masuda et al., 2004).Increasing concentration of HPMC shows release up to 83.77%, 93.10% in 7 and 9 hours respectively.Formulation P4 containing drug: HPMC in ratio 1:3 releases drug up to 97.47%.In vitro release profile is shown in Fig. 2.Among the poloxamer based suppositories tested, the suppositories formulated only with P 188 had significantly lowest dissolution rates for drug (Yong et al., 2001) this may be due to the fact that P 188 remained in solid phase and could not turned into gel in the dissolution medium due to relatively high melting point of P188 (Chul Soon et al., 2005) .So, in this study a combination of two different poloxamer grades was used which resulted in enhancement of dissolution of ondansetron (Choi et al., 1998).To know the mechanism of drug release from these formulations, the data were treated according to zero-order (cumulative amount of drug released versus time), first-order (log cumulative percentage of drug remaining versus time), Highuchi's (cumulative percentage of drug released versus square root of time) model and Peppas-Korsmeyer (log cumulative percentage of drug released versus log time) equations.Poloxamer mixture based suppositories follows zero order release i.e. dissolution rate dependent upon drug concentration with r value in between 0.95 to 0.99.

CONCLUSION
From above studies it can be concluded that suppositories of Ondansetron prepared with different grades of poloxamer as base in combination with HPMC holds potential in patients with chemotherapy induced nausea and vomiting.The prepared poloxamer-based mucoadhesive solid suppositories of ondansetron sustains the release of ondansetron, melts at physiological temperature and has prolonged retention time by adhering to rectal mucosa.

Fig. 2 :
Fig.2:In vitro drug release study of poloxamer mixture based sustained release suppositories of ondansetron.