Abstract
A range of enteric polymers is used in pharmaceutical industry for developing gastro-resistant formulations. It is generally implied that these coatings are interchangeable due to similar dissolution pH thresholds reported by suppliers. Despite rapid dissolution in compendial phosphate buffers, these products can take up to 2 h to disintegrate in-vivo in the human small intestine. The factors primarily responsible for such variability in dissolution of these polymeric coatings are the differences in ionisation of acidic functional groups on polymer chains and their interplay with ions and buffer species present in gastrointestinal fluids. In this study, we aim to develop a novel, simple and inexpensive technique that can be used under various in-vitro conditions to study the ionisation behaviour of commonly used polymers (EUDRAGIT-E100, L100, S100, HPMC AS-LF, AS-HF, HP-50, HP-55) and to estimate their pKa. Moreover, this method was successfully applied to study the ionisation behaviour of a range of natural polymers (Guar, Tara, locust bean, Konjac gums, gum Arabic, citrus pectin, chitosan and alginate) and their pKa was also estimated. The proposed method would allow a better understanding of the dissolution behaviour of these polymers within gastrointestinal tract and will aid rational design of modified release dosage forms.
Highlights
Different types of enteric polymers are used in pharmaceutical industry to develop delayed-release formulations targeting different parts of the gastrointestinal (GI) tract (Fig. 1)
We aimed to develop a simple and economical technique of determining pKa values of various polymeric materials, which can be adopted in different in-vitro conditions to study their ionisation behaviour in a range of pharmaceutical applications, in particular with modified-release dosage forms
The opposite was true for EUDRAGIT E100 which is more extensively ionised at lower pHs, i.e., pH < pKa
Summary
Different types of enteric polymers are used in pharmaceutical industry to develop delayed-release formulations targeting different parts of the gastrointestinal (GI) tract (Fig. 1). It is generally implied, due to their similar dissolution pH thresholds reported by suppliers, that these materials are interchangeable provided the drug release from these products in conventional buffers is similar. According to the Henderson-Hasselbalch equation, the pKa of a weak acid corresponds to the environmental pH at which the concentration of the weak acid ([HA]) equals the concentration of its conjugated base ([A−]). At this pH, the weak acid will tend to partially ionise; whereas almost a full ionisation is expected when the environmental pH is 2-units above its pKa (Fig. 2A)
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