Phil Morton, chief technical officer, Albumedix discusses the effects that albumin can infer on APIs in formulations, and the proposed mechanism behind them.
Recombumin
Development of safe and stable formulations is a challenge of pharmaceutical drug development, and the choice of excipients can become an important factor for their successful formulation. For hard-to-formulate active pharmaceutical ingredients (APIs) this becomes more critical and can be the determinate of whether such a drug candidate gets discontinued in development or becomes a successful pharmaceutical. Albumin has been shown to offer benefits over other excipients in the formulation of various APIs, and can help overcome certain formulation challenges.
Formulating drugs with albumin
Human albumin is the most ubiquitous protein in blood and is present at amounts around 40 g/L. Its role in blood is the shuttling of numerous smaller entities such as metals, hormones, fatty acids and toxins. However, it also makes up about 75% of the colloidal oncotic (or colloidal osmotic) pressure of blood and the single free cysteine of albumin (at position 34) makes up the majority of the reducing equivalents present in blood. All these properties are traits which are functional in employing albumin in formulation.
Albumin has historically been used in a range of different formulations. Originally, plasma sourced human serum albumin was employed, but there has been a shift in the industry towards the use of chemically defined (recombinant) human serum albumin. Recombinant products are advantageous due to factors such as: the absence of animal- derived products, certainty of supply, high purity, absence of host-derived proteases, high homogeneity, high free thiol content, absence of known or unknown human pathogens, batch-to-batch consistency and the presence of an established regulatory pathway.
Albumin in formulation has been reported to prevent:
- Surface adsorption
- Aggregation
- Fibrillation
- Oxidation
And to improve:
- Solubility
- Lyophilised cake formation
- Dissolving properties of API from lyophilised powder.
The mechanistic properties of albumin can be categorised into four benefits:
1.Oxidation protection capability
This stems from the free thiol moiety of cysteine 34 – an unpaired cysteine residue – which is present in its reduced form and can become oxidised instead of the API during oxidative stress. The thiol of cysteine 34 is positioned in a crevice in the albumin protein, which shields it from the solvent and specifically from larger molecules. This confers a relative low reactivity and properties as a scavenger of predominantly highly reactive oxidation species, such as radicals, rather than as a general oxygen scavenger. Cysteine 34 of albumin therefore does not readily become oxidised and can be handled with no specific precautions during formulation process steps.
2. Surface covering ability
Albumin has a predisposition to cover both hydrophilic and hydrophobic surfaces. Historically, albumin has been used to cover the surfaces of medical devices to increase their biocompatibility. Studies have shown spontaneously formed albumin coverage to be in the order of a single albumin molecule in height, with only 1-2 mg albumin needed to cover 1 m2 of surface. This property is utilised for low dose/high potency drugs, where the dose could otherwise be difficult to control due to its surface adsorption during production, handling and storage. Another effect of albumin covering the surfaces is that it can hamper surface-induced unfolding and aggregation events of the API.
3. Direct interaction ability
Albumin’s promiscuous binding properties have resulted in the reporting of numerous APIs binding either in a simple 1:1 ratio, or other more complex ratios. This ability can convey multiple effects on the formulation. For instance albumin can, by direct binding, improve solubility of otherwise difficult to solubilise molecules or prevent otherwise detrimental interactions of molecules that could have given rise to unfolding and aggregation. There are also indications of albumin having the ability to bind to the exposed hydrophobic end of growing fibrils, thereby curtailing their propagation.
4. Indirect interaction ability
Albumin’s positive second viral coefficient means that it will try to avoid interacting with itself in solution, effectively instigating oncotic pressure in blood. The effect also makes for another potential stabilising property of albumin. As the albumin is spaced out evenly in the solution, it hampers the general movement and flexibility of other entities in the solution. The decreased flexibility of the entities in the formulation can translate into increased stability of these entities. The effect on this for formulation is not easily conceived or illustrated, but small-angle X-ray scattering (SAXS) has shown co-formulated proteins to have lower flexibility when formulated with albumin, without there being any direct interaction.
Advancing tomorrow’s formulations
Despite the clear advantages, the use of recombinant proteins as excipients has only seen significant growth relatively recently. Pressure from regulatory bodies and the better understanding of the high-performance capabilities of recombinant human albumin are likely to push these products into the pharmaceutical mainstream. By continuously expanding our knowledge of albumin formulation mechanisms, important but hard-to-formulate APIs will progress more quickly through the pharmaceutical pipeline.