Streamlining HPAPI development

March 20, 2023
How to manage highly potent APIs through a preclinical testing process

High-potency active pharmaceutical ingredients (HPAPIs) are a rapidly growing segment of the pharmaceutical industry. In fact, the $7 billion market for HPAPIs in 2022 is expected to reach almost $15 billion by 2030, led primarily by a growing demand for oncology drugs.1 Higher potency drugs have the potential to achieve similar efficacy at a lower dose than other APIs, which is appealing to drug developers because it minimizes patient exposure to the medication.

HPAPIs make up a significant proportion of new drugs under development. Due to their cytotoxic nature, manufacturers of HPAPIs and the resultant drug products are faced with onerous manufacturing challenges. These drugs require heavy investment to ensure a safe environment for employees. For drug substance manufacturing, the HPAPI may be a small molecule, biologic or a hybrid of the two such as an antibody-drug conjugate (ADC).

Based on therapeutic application, the HPAPI market has been segmented into oncology drugs, anti-diabetic drugs, cardiovascular drugs, CNS drugs, musculoskeletal drugs and other drugs.  The oncology drugs segment has seen a steep rise in the introduction of innovative therapeutic options. ADCs, antibodies and cytotoxic drugs account for the largest share; collectively, they are considered to be the key drivers of the HPAPI market.

With the growing interest in these drugs comes a greater prerogative to properly assess toxicity through preclinical testing.

Defining HPAPIs

Simply put, HPAPIs are pharmaceutical ingredients that have a biological effect at a very low dose. The substance used to develop drugs with HPAPIs does not differ significantly from processes used for less potent pharmaceutical ingredients, but safety concerns arise with manufacturing and handling. Because HPAPIs’ strength presents an inherent potential for cytotoxicity, lab personnel and manufacturing facilities need to be protected. In addition to specialized handling procedures, developing HPAPIs may also require designated equipment to contain, store and manufacture the drug product. These specialized facilities require a significant monetary investment above and beyond what would normally be required under good manufacturing practice (GMP) guidelines.

Traditional examples of HPAPIs are cytotoxic compounds and sex hormones such as estrogen. However, any type of compound can be highly potent if it causes a response at a low dose. HPAPIs can fall into one of five categories:

  • A pharmacologically active ingredient or intermediate with biological activity at approximately 150 µg/kg of body weight or below in humans (therapeutic daily dose at or below 10 mg)
  • An API or intermediate with an Occupational Exposure Limit (OEL) at or below 10 µg/m3 of air as an 8-hour time-weighted average
  • Sex hormones and certain steroids (e.g., corticosteroids)
  • A pharmacologically active ingredients or intermediate with high selectivity (i.e., ability to bind to specific receptors or inhibit specific enzymes) and/or with the potential to cause cancer, mutations, developmental defects or reproductive toxicity at low doses
  • A novel compound of unknown potency and toxicity

These categories span small molecules, large molecules and ADC therapies. The presence of a HPAPIs has an outsized impact on the manufacturing and handling of a drug, but little impact on the safety assessment (including toxicology and drug metabolism and pharmacokinetics studies) that must take place prior to an investigational new drug (IND) submission.

Testing HPAPIs

As a standard practice for safety assessment, animal species selected for toxicity assessment require scientifically sound justification. This is also applicable to designing the toxicology program for HPAPIs. 

Toxicology testing is designed based on the property of the testing material (i.e., small molecule, large molecule or ADC) but not the potency of the active ingredient. To test potency, the following methods are most common:

  • High performance liquid chromatography (HPLC) is used to separate a mixture of compounds to identify, quantify or purify its individual components. HPLC is the preferred method to determine potency in HPAPIs due to its specificity and efficiency. This method can be used to determine drug stability as well, but not predictably. Only a validated stability-indicating method (SIM) can reliably measure an API’s chemical, physical and microbiological properties over time.
  • Titration is a quantitative method that measures an API’s chemical reaction with a known analyte. Titration uses bacteria to assess the antimicrobial activity of an API in relation to a microorganism. These microbial assays measure zones of inhibition, or areas around the spot of the antibiotic in which bacterial colonies do not grow. These zones measure the bacteria’s susceptibility to the antibiotic. A larger zone of inhibition usually means a more potent antimicrobial.
  • Ultraviolet-visible spectrophotometry can be used without chromatography to determine the potency of a single analyte in a solution. By measuring the amount of light absorbed by a chemical substance, UV spectrophotometry can determine concentrations, identify unknown compounds, and provide information about the physical and electronic structures of organic and inorganic compounds. This method is susceptible to erroneous results if compounds interfere with absorption, thus increasing toxicological concern. 

Animal species are then selected based on metabolism profile for a small molecule drug. As such, a series of in vitro and in vivo assays need to be conducted in order to select appropriate animal species that will be employed in the subsequent toxicology studies. In most cases, one rodent and one non-rodent animal species are selected for safety evaluation. On the other hand, only pharmacologically relevant animal species are selected for a biologic toxicity assessment based on binding profile. For example, a mini pig’s skin and heart are similar to a human’s skin and heart, making mini pigs the best choice for testing internally applied compounds. Conversely, a non-human primate’s immune system is closer to a human’s than a mini pig or a canine; thus, they are often the most appropriate species for toxicology testing.

There is no specific regulatory requirement pertaining HPAPIs when designing a toxicology program, but toxicological data plays an important role in establishment of OELs, which subsequently impacts how those active pharmaceutical ingredients are handled further downstream in the manufacturing process.

Balancing benefit and risk

Preclinical testing for drug development, whether synthetic or biologic, is a multistep process requiring scientific expertise and technical experience as well as regulatory guidance. These steps involve multiple studies that can add significantly to drug development timelines, so thoughtful program design is critical to a streamlined submission process.

Specifically, an increased toxicity profile is likely to be associated with an HPAPI due to its high potency. Consequently, toxicity assessment for an HPAPI becomes more challenging than for a less potent API. For example, with a small molecule HPAPI, the efficacy and metabolism profiles are usually established prior to conducting toxicology studies that include (but are not limited to) genotoxicity, and single and repeat dose toxicities in both rodent and non-rodent species.

The drug safety assessment process during preclinical testing can be costly and time-consuming, but ultimately impacts patient safety. Regulators will appreciate preclinical testing outcomes with strong methodology come IND submission.

A final word

There is no denying that HPAPIs pose significant therapeutic promise for cancer and other diseases. But these pharmaceutical ingredients can also pose significant safety risks for patients and those involved in the manufacturing and handling of the drug if they are not properly controlled.

In addition to potency and efficacy, HPAPIs present stability challenges that must be addressed through specific assays designed to alleviate toxicological concern. While a pharmaceutical ingredient’s status as an HPAPI does not directly impact its preclinical testing path, the results of toxicity assessment can relay important information about OELs and other manufacturing and handling limitations ahead of clinical trials.

Preclinical testing for HPAPIs is a lengthy and intricate process, but a quality laboratory testing partner can help mitigate that complexity, potentially streamlining the IND/NDA application process. 

About the Author

Xiaoxia Li | Executive Technical Director, WuXi AppTec