DEVELOPMENT AND VALIDATION OF SPECTROPHOTOMETRIC METHODS
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1.0 INTRODUCTION
1.1 Preamble
Risperidone is a psychotropic (antipsychotic) agent used in the treatment of schizophrenia. The action is mediated through a combination of dopamine Type 2 (D2) and serotonin Type 2 (5HT2) receptor antagonism. It is a selective monoaminergic antagonist with high affinity for 5HT2, D2 and H1 histaminergic receptors (Potter and Hollister, 2001). It belongs to the chemical class of benzisoxazole derivatives. The chemical name of risperidone is 3-[2-[4-(6-fluoro-1, 2-benzisoxazol-3-yl)-1-piperidinyl]ethyl]-6, 7, 8, 9-tetrahydro-2-methyl-4H-pyrido-[1,2-a]-pyrimidin-4-one) while the molecular formula is C23H27FN4O2 with the molecular weight of 410.49g (The Merck Index, 2001).
N CH3
N
N O
O N
Fig 1.1 Chemical structure of risperidone
According to the British Pharmacopoeia (2009), risperidone contains not less than 99.0 per cent and not more than the equivalent of 101.0 per cent of 3-[2-[4-(6-fluoro-1,2-benzisoxazol-3-yl)piperidin-1-yl]ethyl]-2-methyl-6,7,8,9-tetrahydro-4H-pyrido[1,2-a] pyrimidin-4-one, calculated with reference to the dried substance (BP, 2009). The absolute oral bioavailability of risperidone is 70% and a half life of 20 hours. It is rapidly distribute with the volume of distribution being 1-2 L/kg. In plasma, risperidone is bound to albumin. It is extensively metabolized in the liver (USP/NF, 2006). Risperidone was first developed by Janssen-Cilag from 1988 to 1992 and was approved by the Food and Drug Administration in 1994. However, Janssen-Cilag‟s patent on risperidone expired on December 29, 2003 which paved the way for the introduction of cheaper generics into the world market. Some of these cheaper generics, though affordable often fall short of their required efficacy. Simple „on-spot” assessment of these brands of risperidone has therefore become paramount (www.naminh.org).
1.2 Research Problem
The dearth of equipment employed in the determination of risperidone with methods like high performance liquid chromatography (HPLC) (Woestenborghs et al., 1992; Balant-Gorgia et al., 1999; Schatz and Saria, 2000; Zhou et al., 2004; El-Sherif et al., 2005; Huang et al., 2008; Kirschbaum et al., 2008; Baldaniya et al., 2008; Yunoos et al., 2010; Prakash et al., 2014), liquid chromatography (LC) (Avenoso, et al., 2000; Aravagiri and Mander, 2000; McClean et al., 2000; Zhang et al., 2005; Bhatt et al., 2006; Zhang et al., 2007; Locatelli et al., 2009), chemiluminescence assay (Song and Wang, 2004), pulse polarography (Joshi et al., 2006), and the cost of executing these methods constitute an enormous challenge in developing countries like Nigeria. The use of visible spectrophotometric methods reported for the determination of risperidone in its pure form and pharmaceutical preparations have complex procedures and/ or utilization of expensive chemicals and solvents (Hassan, 2008; Narayana and Shetty, 2011; Deepakumari et al, 2013; Archana et al, 2013; Hassouna et al, 2014). This has prompted the call for the development of sensitive, simple and economical ultraviolet-visible (UV)-spectrophotometric methods for the determination of risperidone which can be used to assay risperidone in pharmaceutical formulations available in the market and achieving precise and accurate results with less difficulty and cost.
1.3 Justification of Study
Risperidone has fewer side effects and has benefitted refractory psychotic patients compared to the typical antipsychotics like haloperidol (Shengquan, 2011). This has contributed to its widespread use. In addition, with the introduction of newer and cheaper generics into the market, it is imperative to develop simple, accurate, precise and cost effective methods for the determination of risperidone to ensure routine quality assessment
1.4 Theoretical Framework
The basic nature of the risperidone (pka- 8.24) makes it possible to utilize an anionic dye (bromocresol green and thymol blue) to form an ion-pair complex. Due to resonance effect, protonation of the benzisoxazole ring and pyrimidin-4-one is very difficult. Thus, there is only one site which is susceptible to protonation and that is the nitrogen in the piperidine ring (Harikrishna et al., 2008). Among the two tautomers of the dyes (bromocresol green and thymol blue) present in equilibrium, the quinoid ring must predominate because of the strong acidic nature of the sulfonic group. Finally, protonated risperidone form ion-pairs with the dyes Bromocresol green and Thymol blue in 1:1 ratio. The possible reaction pathway is depicted below
| HO | Br Br | OH | HO | Br Br | O | HO | Br Br | O |
| Br | Br | Br | Br | Br | Br | |||
| H3C | CH3 | H C | H C | + H+ | ||||
| O | 3 | OH | 3 | O | ||||
| S | O | S | O | S– | O | |||
| O | O | O | ||||||
| Lactoid ring | Quinone ring |
1:1 Complex of risperidone–thymol blue
Fig 1.3 Reaction pathway for risperidone-thymol blue complex formation
1.5 Aim and Objectives
1.5.1 Aim
To develop and validate UV spectrophotometric methods for the determination of risperidone in pure and tablet dosage forms
1.5.2 Objective
- Development of UV spectrophotometric methods for the determination of Risperidone.
- Validation of the method developed.
- Application of the developed methods in determination of risperidone in its pure form and in tablet dosage forms.
1.6 Research Hypothesis
An accurate and precise method can be developed for the quantitative determination of risperidone by formation of an ion-pair complex with bromocresol green or thymol blue.