Overcoming the Challenges of Radiolabeling a Gastric Retentive Formulation: Utilizing Polymer Encapsulated Indium-111 Chloride to Assess Gastroretention with Gamma Scintigraphy

Abstract

Purpose. To develop a robust radiolabeling technique to enable evaluation of difficult to radiolabel
gastric retentive formulations using gamma scintigraphy. The use of a successful radiolabel will allow
accurate assessment of the gastric residence time of the formulations.

Materials and Methods. The retention of two radionuclides, indium (111In) and samarium (153Sm), with
and without further processing to improve radiolabel performance were evaluated in simulated gastric
pH in vitro. The most successful formulation from the in vitro screening was further evaluated in
preclinical and clinical studies.

Results. In vitro evaluation revealed significant radionuclide leakage at pH 1.5 for most radiolabeling
attempts. Radionuclide leakage at pH 4.5 was less pronounced. The most successful radiolabel was
formulated by adsorbing indium chloride onto activated charcoal, followed by entrapment in a cellulose
acetate polymer melt. This provided the best radiolabel retention under both pH conditions in vitro. The
radiolabel also proved to be successful during preclinical and clinical evaluations, allowing evaluation of
gastric retention performance as well as complete gastrointestinal transit.

Conclusion. A simple, yet robust radiolabel was developed for gastric retentive formulations to be
evaluated pre-clinically or in a clinical setting by entrapping the radionuclide in an insoluble polymer
through a simple polymer melt process.

Introduction

Gastric retentive formulations (GRFs) have been pursued by both academia and industry for an extensive period of time, due to the clear benefits of such a formulation for drug substances with narrow window of absorption, local treatment or other challenging pharmacokinetic/pharmacodynamic situations. Gastric retentive strategies can be divided into five basic categories: floating, high density,
bioadhesive, large size and gastric motility agents. A significant number of GRFs fall into the category of gastric retention based on a large size (1Y4). However, the size required for gastric retention is not clearly known. Based on endoscopic data from ingestion of large foreign objects and
gastric bezoars, a large, fairly rigid object must be of a size larger than 5 cm in length by 2 cm in diameter to be retained for an extensive period of time in the stomach (5Y7). Endoscopic guidance recommends that if the foreign object does not pose an immediate health risk and if it is smaller
than the previously stated size, a surgical intervention is not required and the object should pass out of the stomach spontaneously (5,6). Although, this information is far from a controlled evaluation of the sizes and mechanical strength required to identify an ideal GRF, it does provide guidance
on the size and the volumetric expansion of the formulation required for gastric retention. To obtain this size (5 cm length2 cm diameter) and be still able to be dosed in a pharmaceutically acceptable format (e.g. 000 capsule) the amount of volumetric swelling needs to be on the order of 15
times the original size. This is quite a formidable task but can be achieved with the proper formulation. The ultimate success of a GRF is based on the achievement of an acceptable pharmacokinetic profile and oral bioavailability of the drug substance; however, alternative approaches can first be applied to determine if a formulation is truly retained in the stomach. Ideally, a noninvasive approach which does not alter the physical properties of the GRF is preferred. Magnetic resonance imaging is gaining popularity in this area (8). Another option is the use of a swallowable camera in the form of a capsule (9)

Background

Gastric retentive formulations (GRFs) are oral dosage forms designed to be retained in the stomach for prolonged periods, as opposed to transiting normally into the small intestine. Prolonged gastric retention can provide therapeutic benefits for certain drugs that have a narrow absorption window in the upper gastrointestinal tract or that are intended to provide localized treatment in the stomach. Some examples include drugs to treat H. pylori infections, drugs where absorption is limited by poor solubility at intestinal pH, and drugs with stability challenges in the intestine.
A critical step in developing GRFs is being able to accurately assess whether the formulation is actually retaining in the stomach as intended over time. Gamma scintigraphy is considered the “gold standard” technique for evaluating gastrointestinal transit and retention of dosage forms in vivo in humans. It involves incorporating a small amount of radionuclide into the formulation which emits gamma radiation that is detected externally by a gamma camera. However, radiolabeling GRFs poses unique challenges because GRFs are purposefully exposed to the low pH gastric environment for prolonged periods, during which the radiolabel may prematurely leak out. This can result in incorrect conclusions about the gastric retention time of the GRF.
This case study examines the development of a customized radiolabeling approach using polymer encapsulated indium-111 chloride to successfully assess retention of a large, swelling gastric retentive formulation in the stomach by gamma scintigraphy. Retention was evaluated in vitro at gastric pH, in vivo in a dog model, and clinically in healthy volunteers.

Methods

The GRF was manufactured using a combination of xanthan gum, locust bean gum, and polyethylene glycol 400 as gelling agents, blended with water. The liquid mixture was poured into molds, allowed to solidify into a gel, and then dried extensively to reduce water content. The dried gel was compressed into large tablets and placed inside size 000 capsules. This created an expandable matrix that was intended to swell and retain in the stomach for an extended duration after oral administration. To identify a suitable radiolabel for the GRF, several candidate radiotracers were screened by measuring retention when incorporated into the GRF matrix and placed in simulated gastric fluid. Retention was evaluated at pH 1.5 to model the fasted stomach environment and pH 4.5 to model fed stomach conditions. Radiolabels tested included:

• Samarium oxide powder and beads
• Indium-111 chloride powder
• Indium-111 chloride adsorbed onto ion exchange resins (Amberjet) or activated charcoal

Indium-111 chloride was ultimately selected due to its half-life of 2.8 days which provides sufficient time to assess long gastric retention. However, simple adsorption of indium-111 chloride powder onto the GRF matrix did not provide adequate radiolabel retention at the low gastric pH.
To improve retention, a polymer encapsulation method was developed where indium-111 chloride was first adsorbed onto activated charcoal particles, followed by melting and blending with cellulose acetate polymer at a 1:6 ratio. The melted mixture was cooled and milled into small microparticles less than 50μm in size, referred to as polymer encapsulated indium-111 chloride (PEIC).
The GRF was radiolabeled by incorporating PEIC microparticles during the manufacturing process. Retention of PEIC in the GRF matrix was evaluated in vitro by measuring radioactivity release over time in simulated gastric fluid at pH 1.5 and 4.5. GRFs were also administered to mongrel dogs and assessed using gamma scintigraphy imaging over 12 hours. Finally, a clinical gamma scintigraphy study was conducted with 6 healthy human volunteers who ingested the PEIC-labeled GRFs after an overnight fast. Anterior and lateral abdominal images were acquired at regular intervals up to 24 hours to assess gastric retention.

Scanning electron microscopy was performed on the PEIC radiolabel at a 500 magnification and b 3,000 magnification of a particle with rougher texture.

PEIC radiolabeled GRF in the stomach of a mongrel dog, 11 h post-dose. Virtually no leakage of the radiolabel was observed. The stomach outline is based on imaging a co-dosed 99mTechnetium labeled solid food.

PEIC radiolabeled GRF in the stomach of healthy human volunteer, 18 h post dose. Virtually no leakage of the radiolabel was observed. The stomach outline is based on imaging a co-dosed 99mTechnetium labeled egg.

Results

In vitro dissolution testing showed that samarium oxide and simple indium chloride powder rapidly leaked out of the GRF matrix at pH 1.5, while retention was improved but still suboptimal at pH 4.5. Adsorbing indium-111 onto ion exchange resins or activated charcoal also did not enhance retention compared to indium chloride alone. In contrast, the polymer encapsulated PEIC radiolabel displayed significantly increased retention at pH 1.5, with an effective half-life of 11.4 hours. At pH 4.5, retention was excellent with an effective half-life of 42 hours. In dogs, gamma scintigraphy images showed that PEIC was stably retained in the GRF with no observable leakage even after 11 hours. Clinically, the GRF was clearly visible in the stomach by gamma scintigraphy for over 24 hours after administration. This demonstrated substantial improvement compared to prior radiolabeling attempts and enabled reliable assessment of gastric retention time.

CONCLUSIONS

Polymer encapsulation of indium-111 chloride enabled successful radiolabeling of a large, swelling gastric retentive formulation for gamma scintigraphy evaluation in vitro, preclinically and clinically. The PEIC radiolabel overcame the challenges of premature radiotracer loss in acidic gastric conditions, which compromised assessment with conventional labeling techniques. Enhanced radiolabel retention was achieved by adsorbing indium-111 chloride onto activated charcoal particles and then coating with cellulose acetate polymer, followed by milling to form microparticles that were incorporated into the GRF matrix. This method can potentially improve assessment of gastric retention for other novel oral dosage forms using gamma scintigraphy.

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Acknowledgements

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