Mike Easterbrook, Chief Technical Officer and Sanjay Negi, DGM- Film Coatings, Ideal Cures, in this study investigate the colour stability, tested under different storage conditions of tablets that were coated with HPMC and HPMC/ Copovidone coating formulations containing indigo carmine
It is a common practice within the pharmaceutical industry to enhance the aesthetic appearance of dosage forms by using suitably coloured film coatings1. Coloured coatings increase patient acceptability and compliance, improve safety via easy identification and differentiation and may reduce degradation via increased opacity to light. Indigo Carmine (FD & C Blue #2) or Indigotine is approved in the US and European markets to be used as a colourant in oral and topical preparations. It is known to have good heat stability but poor stability to light and oxidising agents. Thus formulations containing Indigo Carmine tend to fade with time.
Co-povidone is a one of the polymers which has been used to reduce colour fading issues in film coating systems 2.
The objective of this study was to investigate the colour stability, tested under different storage conditions of tablets that were coated with HPMC and HPMC/ Copovidone coating formulations containing indigo carmine.
Coating formulation preparation
To investigate colour stability differences, HPMC-PEG based and HPMC-Copovidone-based coating systems were prepared. Both formulations contained the same concentrations of Indigo Carmine. Laboratory scale mixers were used to prepare the coating formulations. The ingredients were added and blended together in the mixer until uniform powder blends were obtained.
Evaluation of dry powders
The powders were evaluated visually and samples were retained for stability studies packed in HDPE bottles at different conditions i.e. Ambient (250C± 20C/60 per cent RH±5 per cent) and accelerated stability conditions (400C± 20C/ 75 per cent RH RH±5 per cent) to evaluate the colour fading tendency.
Coating suspension preparation and tablet coating
The coating suspensions were prepared using a laboratory scale stirrer. Plain placebo tablets were coated using these suspensions in a six inch conventional coating pan with pre-determined coating parameters.
Evaluation of coated tablets and stability studies
Coated tablets were evaluated for appearance, colour uniformity, colour difference, disintegration time and coating defects. Tablets coated with both formulations were packed in polyethylene bags within HDPE bottles (screw capped). These were stored for stability studies at the following conditions:
i. Ambient (25ºC± 2ºC/60 per cent RH±5 per cent)
ii. Accelerated (40ºC± 2ºC / 75 per cent RH ±5 per cent)
Open dish samples were exposed to ambient conditions. The tablets were evaluated for appearance and colour difference. Colour difference was checked using a Reflectance Spectrophotometer. The dE values were measured over a six-month period to evaluate colour fading over time.
Results and discussion
The HPMC-PEG and HPMC-Copovidone-based formulations were prepared using a laboratory scale mixer. The plain placebo tablets were coated with both the formulations using the standard coating process parameters recommended for a HPMC-based coating formulation. The coated tablets were evaluated for physical characteristics and found comparable. All samples stored under all conditions showed some colour fading with time as shown in the photographs at Table No. 1. Colour differences between coated tablets stored under ambient and accelerated conditions for different time periods were measured using the Reflectance Spectrophotometer and expressed in terms of total colour difference (dE) with respect to initial samples. Copovidone-based formulations exhibited good colour stability at all the stability conditions as compared to other HPMC-based formulation.
From the findings of this study, colour fading was reduced with HPMC/ Copovidone coating formulations compared with a standard HPMC-based coating system. It is suggested that product stability may be improved by including copovidone in the coating formulations.
1. Colorants-The Cosmetics for the Pharmaceutical Dosage Forms, Krishna Vamshi Allam, Gannu Praveen Kumar, Int J Pharm Pharm Sci, Vol 3, Suppl 3, 2011, Pg.No.13-21.
2. Moroni A.et.al, Pharmaceutical Tablet Coating Composition, US Patent 20040001884A1, January 1 2014.