David O’Connell, director of scientific and technical affairs, PCI Pharma explores the rise of ultra-high potency APIs and the limits of traditional HPAPI definitions.
PCI
For many years, “high potency” has been the industry’s catch‑all for active pharmaceutical ingredients (APIs) that demand specialised handling, stringent engineering controls and risk‑based containment measures, enshrined in the now‑familiar acronym HPAPI. This label has become embedded in regulatory guidance, internal SOPs, facility design and outsourcing contracts, but it increasingly fails to describe the realities of today’s most powerful small‑molecule therapies.
A growing subset of targeted protein degraders (TPDs) such as PROTACs, SERDs and molecular glues, together with certain ADC payloads, display orders‑of‑magnitude higher potency, very narrow therapeutic indices and complex toxicology that stretch traditional HPAPI assumptions. Their behaviour often lies beyond the design space of legacy high‑potent plants, forcing pharma companies and CDMOs to revisit how they classify and control risk. In response, the informal term “ultra‑high potency” has begun to surface as a pragmatic way to signal that a compound sits at the very top of the potency spectrum, even if regulators have not yet defined it formally.
When one category covers too much ground
Despite its ubiquity, HPAPI still lacks a harmonised global definition, so organisations typically lean on long‑standing benchmarks such as doses at or below 1 mg/day, OELs at or below 10 μg/m³ and toxicological profiles where small deviations in exposure can be clinically significant. By these criteria, an estimated 30–50% of new small‑molecule candidates now fall into the high‑potency bucket, creating a broad class that can mask major differences in hazard and handling complexity. A cytotoxic with an OEL near 5 μg/m³ may be grouped alongside a TPD that ultimately proves to require sub‑microgram or even sub‑nanogram‑level containment, yet both are simply labelled “HPAPI”.
For many emerging modalities, formal OELs may not be fully defined, but features such as catalytic mechanisms, ultra‑low therapeutic doses and evolving safety profiles clearly point to a need for much tighter containment than that associated with historical high‑potent platforms. The resulting mismatch between language and reality clouds risk assessment, complicates facility selection and undermines programme planning, particularly as more assets drift into the highest occupational exposure bands.
Defining “ultra-high potency” in practice
In day‑to‑day use, “ultra‑high potency” has become a functional label for molecules assigned to the upper OEB tiers, typically OEB 5–6, where expected OELs drop below 1 μg/m³ and in some instances reach the 0.1–0.01 μg/m³ range. The distinction is not only about sheer potency but also about biological behaviour: many of these agents drive protein degradation or multi‑pathway signalling, making off‑target effects harder to predict and prompting more conservative worker‑protection strategies. Their extremely low doses tighten process tolerances, increase the consequences of cross‑contamination and demand highly sensitive analytical methods, enhanced cleaning regimes and stricter segregation to maintain control.
The term has therefore emerged because these assets simply exceed the assumptions implied by the traditional HPAPI designation, particularly for smaller innovators who may only discover that a compound resides in this “upper tier” after detailed risk assessment. Once that happens, the implications are immediate for formulation decisions, containment strategy and overall programme feasibility.
Technical implications across the development lifecycle
Ultra‑high potency status drives more rigorous technical planning from the outset, since these products typically require pristine, multilayered high‑containment manufacturing environments. Many facilities marketed as HPAPI‑capable were built around mid‑range potency expectations and may lack rigid‑wall isolators, fully closed transfers, engineered airflow and segregation robust enough for sub‑microgram OELs, gaps that often emerge only when full risk assessments are performed. At the same time, cleaning validation, environmental monitoring and residue analysis must function at far lower detection limits, calling for state‑of‑the‑art analytical platforms, specialised method development and expertise that sponsors may need earlier than they anticipate.
Process windows are typically narrower, with micro‑batches, miniaturised or bespoke equipment and higher levels of automation used to maintain containment and dose uniformity, all of which add complexity and cost for resource‑constrained organisations. The stakes are also higher from a regulatory and reputational perspective, so developers must understand a CDMO’s OEB assignment process, containment‑verification history and environmental monitoring performance before entrusting ultra‑potent assets to a partner.
What this means for CDMOs and sponsors
For outsourcing providers, the emergence of “ultra‑high potency” underscores a widening capabilities gap between routine high‑potent operations and those able to support early‑stage work, scale‑up and clinical supply at validated OEB 5–6 levels. Simply advertising HPAPI capability no longer suffices; sponsors increasingly expect transparency around which OEB bands a site can support, how containment performance has been demonstrated and how frequently it is re‑verified.
Facilities handling ultra‑potent molecules need isolators qualified to extremely low OELs, integrated and automated material transfers, engineered containment systems and continuous particulate monitoring, underpinned by rigorous maintenance and requalification. They also require advanced analytical infrastructure with very low limits of detection and a workforce steeped in toxicology, disciplined training and a culture in which deviation‑free execution is a baseline expectation rather than an aspiration.
Whether or not “ultra‑high potency” ever becomes a formal regulatory category, its growing use simply reflects scientific progress and highlights the need for a more nuanced, shared vocabulary to guide partners toward facilities genuinely equipped for the uppermost potency bands.