At the outset of the formulation development, we were presented with a range of non-proprietary formulation options, based mainly on conventional multi-particulate technologies, as well as proprietary, patented, formulation options that had claimed specificity for circadian drug delivery. These proprietary technologies shared a common drug delivery objective of conferring both delayed- and sustained-release functionality in one dosage form (Table 2). The claimed mechanisms for drug release, whilst not specifically disclosed in the scientific literature, were centred on established principles of time-dependent polymer erosion, pH-triggered polymer dissolution, osmotic pressure activation and/or diffusional transport. Despite the overt multiplicity of formulation options, few had been evaluated in the clinical setting, and even fewer had been directed towards the circadian delivery of endogenous hormones, with the only exception being the Egalet system, which was used to test the sustained-release delivery for hydrocortisone.
We initially partnered with a company specialising in electrostatic deposition technology (Phoqus Pharmaceuticals Limited, West Malling, UK) to develop a proprietary circadian formulation for hydrocortisone. The electrostatic deposition technology, at that time, was highly differentiated, as it was the only drug delivery technology that could deposit a precise polymer coating on selective surfaces of a tablet formulation to enable the attainment of both delayed- and sustained-release functionality. The only comparable technology to electrostatic deposition was a press-coated tablet (Geoclock®, developed by SkyPharma) utilising compression (press) coating to achieve partial tablet coating, which was applied to the circadian delivery of prednisolone (Lodotra®). The principal drawback of press-coating was the inherent difficulty in controlling the uniformity and thickness of the compression coating to minimise pharmacokinetic variations, an aspect which was overcome using the electrostatic deposition technology.
The hydrocortisone circadian dosage form developed using electrostatic deposition was a bilayer modified-release tablet encased with a water-insoluble barrier coating (ammonio methacrylate copolymer, type B) to all but one face of the tablet. The bilayer comprised an active layer containing hydrocortisone and rate-controlling polymers: carbomer homopolymer, type A, and ammonia methacrylate copolymer, type B, surrounded by the barrier coat. The adjoining eroding layer contained an enteric polymer: methacrylic acid copolymer, type A, also surrounded by the barrier coat but with an uncoated exposed surface. The eroding layer was designed to confer delayed-release by preventing the ingress of water into the active layer until complete erosion of the eroding layer had occurred. The duration of delayed-release could thus be optimised by altering the thickness of the eroding layer. The subsequent release rate of hydrocortisone from the active layer could also be independently controlled by altering the polymer concentration so as to alter the diffusional transport of hydrocortisone.
Prototype formulations were developed using electrostatic deposition targeting a delayed-release period of around 3–5 h, followed by a sustained-release period of c. 16–20 h, based on Gastroplus® in silico modelling to achieve close mimicry of the circadian pattern of cortisol. Pharmacokinetic evaluation of two early prototype formulations at a dose of 30 mg in healthy volunteers  showed excellent mimicry of the early phase of the circadian pattern of cortisol; however, the level of serum hydrocortisone showed gradual but notable decline from c. 08:00 h, limiting coverage beyond mid-morning.
Follow-on formulations were developed with Phoqus, to refine the release of hydrocortisone, principally by extending the delayed-release duration (from 2 to 4 h) and shortening the sustained-release phase (from 16 to 12 h) to achieve improved serum levels of hydrocortisone from mid-morning onwards. Phase I, pharmacokinetic evaluation demonstrated again good mimicry of the early phase of the circadian pattern of cortisol and a slight improvement to the serum levels of hydrocortisone for the morning exposure compared to the original formulations. In phase I, the exposure of hydrocortisone from these formulations was approximately dose proportional, indicating the absence of dose-limiting absorption.
Formulations were further evaluated in a phase IIa study , in patients with CAH to compare the steady-state serum concentration profiles and pharmacodynamic responses relative to immediate-release hydrocortisone (Cortef®, Pfizer, USA). The phase IIa study confirmed that the formulations could reproduce the overnight cortisol rise and could improve biochemical control of CAH in the morning. The hurdle of achieving improved levels of serum hydrocortisone from mid-morning onwards for once-a-day dosing was not satisfactorily concluded as Phoqus Pharmaceuticals had ceased its operation for financial reasons, with no prospect for continued development of the electrostatic deposition technology.
A key learning point from the electrostatic deposition technology experience was that combining a new therapeutic (circadian delivery) concept with a new formulation drug delivery approach greatly increased the risk of failure with two unproven approaches. With that experience, we then turned to the use of a conventional modified-release technology platform, multi-particulates using drug and polymer-layering, to integrate the delayed- and sustained-release features into a revised circadian formulation of hydrocortisone. The key advantage of the multi-particulate technology is that with judicious formulation design, the delayed-release functionality can be controlled independently from the sustained-release functionality, thereby providing scope to optimise the pharmacokinetic profile of hydrocortisone against the circadian pattern of cortisol. The architecture of the base multi-particulate formulation comprised a microcrystalline core layered with hydrocortisone, a sustained-release coat of ammonia methacrylate copolymer, type A and type B, and an outer enteric polymer coat of methacrylic acid-methyl methacrylate copolymer (Fig. 3a).
Although initial in vitro correlation against the selected target product profile was promising (Fig. 3b), pharmacokinetic evaluation in healthy volunteers of a prototype multi-particulate formulation (DIURF-000) with a designed delayed-release duration of 3 to 5 h and a sustained-release duration of 18 h, akin to the original Phoqus Formulation, proved disappointing, marked by variability in drug release onset and generally low serum levels of hydrocortisone caused by a significant shortfall in relative bioavailability (~ 34%). Unlike the Phoqus formulation, which appeared to transit the gastrointestinal (GI) tract slower and more predictably than multi-particulates, the rapid transit of the multi-particulates we presumed had resulted in the majority of the drug payload being delivered to the colon, a region of the GI tract wherein the absorption of hydrocortisone had not been formally studied. A working hypothesis was that hydrocortisone was poorly absorbed in the colon, possibly due to its very low aqueous solubility and limited colonic permeation. In view of the poor bioavailability, three further multi-particulate formulations were developed, all with reduced thickness of the sustained-release coat. Relative bioavailability was improved but remained < 50% compared to immediate-release hydrocortisone, and the shape of the pharmacokinetic profile was similar to that for DIURF-000.
A series of investigative studies were conducted in dogs to determine the optimal ‘absorption window’ for hydrocortisone, enabled by multi-particulate formulations with a significantly shortened sustained-release phase such that the majority of the drug payload would be released in the small intestine. These studies provided definitive proof that the absorption window for hydrocortisone was limited to principally the small intestine and that sustained-release may not be an essential design attribute as hydrocortisone appeared to exhibit dissolution rate-limiting absorption by virtue of its very low aqueous solubility .
The multi-particulate formulation was re-designed to significantly reduce the sustained-release component, with one formulation variant being devoid of any sustained-release coating. The re-designed multi-particulate formulations were evaluated in an extended phase I study , in healthy volunteers with interim pharmacokinetic readout to permit a revision to the dosing regimen for optimal mimicry of the circadian pattern for cortisol. The results of the pharmacokinetic study correlated remarkably well with the dog study, confirming the selective absorption window for hydrocortisone and verifying critically that sustained-release functionality was indeed not necessary as the intrinsic dissolution rate of hydrocortisone was already sufficiently slow to maintain an absorption input rate that was commensurate with the required target pharmacokinetic profile.
Given the absorption window constraints for hydrocortisone, it was considered not plausible to utilise absorption in the colon to facilitate extended delivery for hydrocortisone, for once-a-day dosing. An alternative approach was adopted using twice-daily dosing via a ‘toothbrush’ regimen to optimise patient compliance, i.e. dosing at night before bedtime and in the morning on waking, to provide full 24-h coverage of hydrocortisone following the circadian pattern of cortisol. This dosing regimen (20 mg at 23:00 h and 10 mg at 07:00 h) was evaluated in the extended pharmacokinetic study in healthy volunteers and was shown to provide consistent physiological levels of hydrocortisone with similar relatively bioavailability to immediate-release hydrocortisone, and the serum concentration was observed to increase linearly with doses between 5 and 30 mg . The formulation used in this study was progressed for clinical development.