Bioavailability of therapeutic proteins by inhalation--worker safety aspects.

A literature review and analysis of inhalation bioavailability data for large therapeutic proteins was conducted in order to develop a practical estimate of the inhalation bioavailability of these drugs. This value is incorporated into equations used to derive occupational exposure limits(OELs) to protect biopharmaceutical manufacturing workers from systemic effects. Descriptive statistics implies that a value of 0.05, or 5% is an accurate estimate for large therapeutic proteins (molecular weight ≥ 40 kDa). This estimate is confirmed by pharmacokinetic modeling of data from a human daily repeat-dose inhalation study of immunoglobulin G. In conclusion, we recommend using 5% bioavailability by inhalation when developing OELs for large therapeutic proteins. K e y w o r d s : inhalation bioavailability; occupational exposure limit; OEL; therapeutic proteins I n t r o d u c t I o n Biologics constituted >30% of approved pharmaceuticals, with 179 new biologics (that are not vaccines or blood products) approved between 1993 and early 2013 (Sathish et al., 2013; U. S. FDA, 2013). These drugs have offered novel and effective treatments for immune-mediated inflammatory diseases, infection, hematology, and a variety of cancers. This class of molecules includes various recombinant proteins, fusion proteins, and monoclonal antibodies that are being developed to treat these various diseases. They are large chains of amino acids that are produced through expression in biological organisms (typically Chinese hamster ovary cells, or Escherichia coli bacteria) instead of synthetic organic chemical manufacturing. Because amino acids are naturally occurring, mostly in dilute solution, and the studies to date demonstrate poor inhalation bioavailability as a drug delivery route, there is thought that proteins pose a much lower risk to workers handling them in the workplace. However, these are still very potent, pharmacological agents when delivered by the therapeutic Ann. Occup. Hyg., 2014, Vol. 58, No. 7, 899–911 doi:10.1093/annhyg/meu038 Advance Access publication 23 June 2014 route of administration (e.g. intravenous or subcutaneous injection). Any pharmacologic effects that may occur following protein inhalation are relevant to setting occupational exposure limits (OELs). In a review, pharmacologically relevant absorption through the airways was reported for the relatively small proteins and peptides like insulin, calcitonin, growth factors, alpha-1-antitrypsin, luteinizing hormone-releasing agonists and antagonists, vasopressin analog, interferons, and granulocyte colony-stimulating factor, and also for larger proteins, namely immunoglobulins (Ig), in animals and limited clinical studies (Agu et al., 2001). Thus, the risk from inhaled proteins may not be presumed to be zero. Biopharmaceutical workers are involved with large manufacturing scale production of these proteins. Consequently, occupational health professionals need to ensure that both the pharmacological and toxicological effects of these drugs are prevented among workers handling these materials. Companies and government agencies accomplish this by setting workplace OELs. To develop an OEL, nonclinical toxicological, pharmacological, and pharmacokinetic (PK) data along with clinical pharmacodynamic, PK, and safety data are reviewed. The risk assessment models for OELs use multiple uncertainty factors applied to a Point of Departure (POD), a No Observed Adverse Effect Level (NOAEL), or Lowest Observed Adverse Effect Level (LOAEL) for the most sensitive endpoint (the critical effect) in the most relevant species (Naumann and Sargent, 1997; EPA, 2011; ICH, 2011). Due to the extensive datasets required for pharmaceuticals, chemical-specific adjustment factors (uncertainty and PK factors) to the POD may be used (Sargent and Kirk, 1988; Silverman et al., 1999; IPCS, 2001). There is some suggestion that using the traditional small molecule approach may be overly conservative for protein therapeutics. First, the preclinical and clinical study designs for patient safety are via parenteral injection (typically intravenous or subcutaneous administration) with intermittent dosing. This gives a clear blood exposure that equates to a pharmacological and/or toxicological effect. In contrast, the usual route-to-route adjustment in the OEL calculation is generally assumed to be 100% bioavailability for small molecule pharmaceuticals when inhalation bioavailability or toxicity data are not available. This inhalation bioavailability assumption would significantly overestimate the exposure for a therapeutic protein. There are biological reasons to expect that the ability of an inhaled protein to enter the systemic circulation is limited. Therefore, an adjustment to the route-toroute bioavailability factor in the calculation may be appropriate. We have performed an analysis of published data related to the inhalation bioavailability of therapeutic proteins in order to estimate a more accurate correction factor for use when calculating OEL for biopharmaceutical manufacturing. Absorption and facilitation of proteins through airways and alveoli The lung is naturally permeable to a number of therapeutic peptides and proteins, and far more permeable to proteins than any other portal of entry into the body. The large [80–120 m2, up to 140 m2 (Witschi and Last, 2001)] absorptive surface of the lung is covered by an extremely thin layer of fluid (volume 10–30 ml) (Patton et al., 2010). Thus, an inhaled aerosol can be dispersed and deposited in quite high concentrations in close proximity to the bloodstream. Moreover, the surface fluids of the lung contain antiproteases that inhibit the enzymatic breakdown of proteins. Unlike the nasal passages and gastrointestinal tract, where lateral movement of bulk fluid occurs, the alveoli of the deep lung are cul-de-sacs where residence times of molecules at the absorptive surface may be prolonged (Patton et al., 2004). ‘Blocked’ peptides like cyclosporin, peptides that have been chemically altered to protect them from degradation by peptidase enzymes exhibit very high bioavailability by the pulmonary route compared to the oral and dermal routes (ibid). In general, proteins with molecular weights (MW) between 6 and 50 kDa are relatively resistant to most peptidases and have a relatively high bioavailability following inhalation when compared to larger proteins. However, aggregation of inhaled proteins stimulates opsonization (coating) by surfactant and by proteins suspended in the lung fluids. This way, the aggregated proteins become marked for phagocytosis and intracellular enzymatic destruction (Moller et al., 2008). Facilitation of absorption of proteins through airways and alveoli For most proteins to reach the circulation, the material must first reach the deep lung. This is primarily 900 • Bioavailability of therapeutic proteins by inhalation