Formulation development is a long, ongoing process during a candidate’s lifecycle. Broadly, it’s segmented into three categories: pre-formulation, formulation, and process development. The work of pre-formulations groups is to begin the stability-indicating assays, and identify (and sometimes finalize) lead excipients. Their goal is to find optimal conditions to keep new biologic drug candidates soluble and active for further study, and eventually scale-up. They work to avoid protein aggregation or protein denaturation with the right pH, salt concentrations, and excipients.
The work of pre-formulations scientists is critical to de-risk the entire development process for biologics. Read on to learn more about what to consider about your buffer components for pre-formulation biologics optimization.
1. pH/buffering
One of the key considerations of a biologic formulation is its pH. The pH of the buffer solution will influence the surface charge of a protein being used in therapeutics, which in turn influences how well the protein stays soluble and un-aggregated in solution. However, it’s also important to consider administration – most biologics are administered intravenously or subcutaneously, which means they must be formulated for delivery at or near blood pH of about 7.35-7.45.
Antibodies often demonstrate better colloidal stability at lower pHs – as shown in this application note – so balancing the need for biologically-relevant pH levels with those needed for improved long-term storage capacity is a critical consideration.
The pKa of a buffer component determines the range in which it can functionally act to control the final pH of the solution. Buffers work within +/-1 pH unit of their pKa. Some of the most common buffer salts include Tris (pKa = 8.1), Phosphate buffered saline (usually formulated at pH 7.4), histidine (pKa = 6.01), and citrate (pKa3 = 6.04).
2. Salt
Buffer salts alone are usually not enough to keep a protein-based therapeutic soluble. Free ions give the termini and ionized side chains of a protein something to bind to besides each other, resulting in reduced aggregation propensity from attractive forces between protein molecules. Salts are also critical to maintain blood homeostasis during delivery of the therapeutic, and they impact the conductivity of a solution, which is important for reproducible chromatography processes.
Sodium chloride (NaCl) and potassium chloride (KCl) are among the most common salts added to maintain conductivity and solubilization of biologics. It is also important to consider how conjugate salts from other additives will influence overall salt concentration.
3. Excipients
For administration, there are often additional additives that help improve the efficacy of a biologic, whether by boosting its effect or by keeping it stable for longer. These are called excipients, and they are any component added to the final formulation of a treatment that isn’t part of the active drug. Some examples include adjuvants, surfactants, polyols, sugars, and amino acids [read here to learn more about these excipients and how they help enhance the stability of biologics].
4. Downstream assay requirements
Most considerations listed above come from the perspective of storage, where the concern is keeping a biologic stable long term for shipment and storage in a clinic prior to administration. It’s also worth noting, for distribution after manufacture proteins are often lyophilized. However, pre-formulations work is often focused on optimizing aqueous therapeutics, as it occurs before the biologic candidate has been fully characterized, including cellular assays, mouse studies, and other activity assays.
Each additional assay required to build a full developability profile for a candidate may have its own buffer requirements, which in turn will impact stability. It’s important to test your candidate’s stability in these other assay buffers, and modify if necessary to retain the form and function of your biologic.
Watch this webinar for more information on how stability screening impacts the building of developability profiles for your biologic candidates.
5. Material cost
If you have a lot of buffers to screen, the process becomes very time-, labour-, and material-intensive. There are two ways to cut down on the cost of buffer screening.
The first is to reduce the amount of sample used to get stability information. With an instrument that gives you more data about your biologic’s stability and with a lower limit of detection, you’ll get more information to make decisions about buffer optimization without having to scale up too extensively early in your development process.
Second, as you design an optimal buffer from this stability information, consider whether it’s worth sacrificing a little bit of stability for a big savings in manufacture. As production scales, a few cents difference in buffer components has a big impact on the final cost of the therapeutic. For example, a buffer made with HEPES costs about $5/L whereas when made with PBS would cost about $0.40/L. If the stability impact is minimal, swapping out a less expensive component is likely better economically.
Conclusion
These are only a few considerations for buffer formulation for biologics. Each therapeutic behaves differently, and it’s critical to do this buffer optimization work for better clinical success.
Tools such as the Prometheus Panta help upstream researchers in the pre-formulations space make assessments about how buffer components improve the stability of their therapeutics. High-resolution data on multiple stability parameters with low limits of detection help accelerate the buffer optimization process.
Careful consideration of the impact of pH, salt concentration, and other excipients will set your biologic candidates up for success.
