The Double Problem
The molecules that generate the most clinical excitement are often the ones that cause the most formulation headaches. Modern oncology APIs, for example, tend to be both highly potent (small doses, strict containment requirements) and poorly soluble (BCS Class II or IV, significant bioavailability challenges). Solving one problem in isolation is hard enough. Solving both simultaneously requires a formulation strategy that navigates containment requirements and bioavailability enhancement at the same time.
This article looks at how those two challenges interact and how development teams can avoid designing a solution for one that undermines the other.
Where the Challenges Collide
Spray Drying with HPAPIs
Spray drying is one of the most effective routes to an amorphous solid dispersion for a poorly soluble API. For a high-potency compound, it also generates an aerosol of fine particles during atomisation, exactly the exposure route that makes HPAPIs dangerous. The spray drying chamber needs engineering controls appropriate to the OEB classification of the API, and the secondary drying step, which typically involves fluid bed processing, must also be conducted within a contained environment.
This is not insurmountable, but it requires a facility where spray drying equipment is integrated with HPAPI containment infrastructure. A site that has spray drying and a site that has HPAPI containment are not the same thing as a site that has both in the same building.
Wet Milling with HPAPIs
Wet milling for nanosuspension manufacture is lower-risk from a containment perspective because the API is suspended in liquid throughout the milling process, reducing the risk of airborne exposure. However, the downstream steps, particularly spray drying or fluid bed granulation of the milled suspension, reintroduce aerosol risk. Closed transfer systems and contained fluid bed processing are standard mitigations.
HME with HPAPIs
Hot melt extrusion produces a solid extrudate rather than an aerosol, which gives it an inherent containment advantage over spray drying. The milling of the cooled extrudate to a usable particle size does generate dust, which requires containment. For many HPAPIs, HME followed by contained milling is a more practical manufacturing route than spray drying, provided the API is thermally stable at processing temperatures.
Formulation Technology Options for High Potency, Low Solubility APIs
| Technology | Bioavailability Benefit | HPAPI Containment Considerations | Best Suited For |
| Hot melt extrusion (HME) | ASD formation; 2-10 fold AUC improvement | Contained milling of extrudate required; lower aerosol risk than spray drying | Thermally stable HPAPIs; OEB 3-4 |
| Spray dried dispersion | ASD formation; similar bioavailability benefit to HME | Contained spray dryer and secondary drying required; aerosol generation in chamber | Thermally labile HPAPIs where HME is not feasible; requires dedicated HPAPI spray drying facility |
| Nanosuspension (wet milling) | 2-5 fold dissolution rate improvement; crystalline API retained | Closed milling system; contained downstream processing | HPAPIs where crystalline form is required; OEB 3-4 |
| Lipid-based formulation (SMEDDS) | Enhanced solubilisation via self-emulsification | Liquid handling; lower aerosol risk; containment simpler | Lipophilic HPAPIs; fill-in-capsule formulations |
| Cyclodextrin complexation | Solubility improvement via inclusion complex | Aqueous processing; low aerosol risk | Moderate potency; HPAPIs with suitable cavity fit for cyclodextrin complexation |
Dose Accuracy: The Third Challenge
For HPAPIs dosed at microgram or sub-milligram levels, the analytical challenge of confirming content uniformity is significant. Standard HPLC-UV methods may lack the sensitivity needed at the concentrations involved. LC-MS/MS methods with lower limits of detection in the nanogram range are often required for content uniformity and dissolution testing of HPAPI drug products.
This analytical requirement must be built into the development plan early. A formulation team that spends six months developing an ASD strategy, then discovers the release method cannot measure the drug at the target dose, has a problem that is expensive to solve retrospectively.
Ardena’s HPAPI Formulation Capabilities at Pamplona
Ardena’s Pamplona (Idifarma) facility combines OEB 3, 4, and 5 containment capability with formulation development infrastructure for poorly soluble APIs, including HME, wet milling, and lipid-based formulation platforms. The analytical team at Pamplona develops high-sensitivity LC-MS/MS methods for content uniformity and dissolution testing of HPAPI drug products as an integrated part of the formulation development programme.
