Pathogen Reduction in Platelets: A Review of the Proposed Draft Guidance

Current Recommendations for Platelets

The AABB (formerly, the American Association of Blood Banks) recommends prophylactic transfusion to reduce the risk for spontaneous bleeding in patients who have a platelet count of 10 ×10⁹ cell/L or less.⁴ Other indications include decreased platelet function due to drug therapy, myeloproliferative disease, or active hemorrhage. As platelet usage increases, medical directors, physicians, and transfusion services personnel struggle to maintain a safe and adequate inventory of platelets. Confounding this struggle are the complications that arise due to the current storage requirement of platelets. Current standards require platelet products to be stored at room temperature,⁴ which increases the likelihood of bacterial proliferation in platelet products.⁵ To prevent bacterial proliferation in these units, donor centers are required to hold platelets for bacterial testing for a minimum of two days before they are released to transfusion services (TS). Once the platelets arrive at the TS, they typically expire in three days or less. The short expiration time challenges hospitals and transfusion services that try to maintain adequate inventories of platelets.

Despite these bacterial testing requirements, the FDA reports 13 fatalities from 2009 to 2013 as a result of bacterial contamination of platelet products.⁶ According to a recent study conducted by Hong, Xiao, Lazares et al.,² this number may be underestimated by as much as 10-fold. Some recognition criteria of septic transfusion reactions (STRs) overlap with criteria for noninfectious febrile/febrile-like transfusion reactions,² which can lead to possible misdiagnoses. The authors assert that more measures should be taken to address this problem.

Proposed Recommendations for Platelets

Cold temperature (4 °C) storage has been proposed to reduce pathogens in platelet products.^7,8 While some studies have shown that platelet function can be preserved at cold temperatures (4 °C),^7,8 there is currently no globally accepted test to demonstrate platelet function in vivo.¹ Unfortunately, von Willenbrand factor receptors can aggregate at cold temperatures leading to the phagocytosis of platelets by liver macrophages in vivo.¹ Therefore, an alternative method is required to reduce TTIs of platelet products. In an effort to reduce TTIs due to platelet products, CBER released its Draft Guidance for industry entitled, “Bacterial Risk Control Strategies for Blood Collections Establishments and Transfusion Services to Enhance the Safety and Availability of Platelets for Transfusion,” in March 2016. To supplement initial bacterial detection and culture as previously described³, the FDA now recommends additional measures to reduce bacterial contamination of platelet products. The first method described in the CBER draft guidance is a rapid bacterial detection test performed on day four- or day five-stored platelets, prior to transfusion. This rapid detection test is recommended even when there is no growth of bacteria in the platelet cultures that were performed on day one after collection. Rapid bacterial detection tests should be conducted within 24 hours prior to transfusion. According to the Blood Products Advisory Committee in 2012, transfusion of platelets that have been stored for 4 or 5 days have been the source of all fatalities due to bacterial sepsis and most (95%) of platelet transfusion-related septic reactions.⁹ To minimize TTIs, then, it would seem that shortening the duration of platelet storage to 3 or 4 days maximum would be the logical choice, but that would result in significantly decreased platelet availability, and is therefore not considered to be a feasible strategy.

An alternative recommendation by the FDA is the use of Pathogen-Reduction Technology (PRT). While solvent-detergent methods are currently in place for the use of pathogen reduction in plasma and non-cellular blood products,¹ PRT in platelets inactivates viruses, fungi, bacteria, and parasites within blood components that contain cells using a photochemical process.¹⁰ The photochemical treatment prevents the replication of virtually all microorganisms, while preserving cellular function and minimizing toxicity of the treated platelets.

Technologies for Rapid Bacterial Detection

Currently, there are two rapid bacterial detection tests cleared by the FDA for bacterial detection in platelet products. The BacTx (Immunetics, Inc., Boston, MA), detects peptidoglycan that is found exclusively in bacterial cell walls.¹⁰ Bacterial contamination triggers a melanization reaction, triggering an enzyme cascade and resulting in melanin formation. After 30 minutes, a photometric change in color represents a positive bacterial detection.¹¹ A study conducted by Heaton, Good, Galloway-Haskins et al.,¹² demonstrated sensitivity ranging from 6.3 × 10² CFUs/mL for Staphyloccocus. epidermidis to 7.6 × 10⁴ CFUs/mL for Eschericia coli. The sensitivity for remaining organisms was less than 7.6 × 10⁴, all of which are below the clinically significant level of 10⁵ CFUs/mL.

The platelet Pan Genera Detection (PGD) test (Verax Biomedical, Inc., Worcester, MA) uses an enzyme-linked immunoabsorbent assay to detect the lipoteichoic acid of gram-positive bacteria and polysaccharide antigens specific to gram-negative bacteria.¹³ It is the only rapid bacterial detection test cleared for use as a “safety measure” by the FDA. A test can be labeled for use as a safety measure when “clinical studies have shown benefit for detection of contamination not revealed by previous bacterial testing and where clinical specificity was determined.”³ According to the draft guidance, these two approved tests act as a safety measure and can extend the shelf life of platelet products by an additional 24 hours, not to exceed 7 days.

Benefits of both rapid bacterial methods are clear. Both detection methods can detect clinically significant levels of bacterial contamination of platelets, serving as an additional safety measure to reduce TTIs and STRs. Neither methodology has been shown to alter or interfere with the efficacy of the platelet product. In addition, the PGD test allows for an extension of platelet shelf life, aiding in inventory management and reducing platelet wastage. However, the PGD test is not currently cleared for use on platelets suspended in platelet-additive solutions (PAS). Neither test detects viral or protozoan activity in platelet products. In addition, neither bacterial detection test is cleared for reduction of any additional adverse platelet transfusion events.

Pathogen Reductions Technology

Pathogen reduction technology (PRT) is currently cleared to control bacterial contamination risk. The Intercept® system utilizes amotosalen in the presence of UV light.¹⁰ Amotosalen is a psoralen that selectively binds to nucleic acids cross-linking them upon photoactivation, rendering pathogens inactive, and unable to replicate.¹⁰ The Intercept brand includes a compound absorption device to recover residual amotosalen to reduce toxicity of the treated platelet product.

Although the Intercept® system package insert notes that there is about a 10% loss of platelet product when undergoing amotosalen-UV treatment,¹¹ efficacy of the platelet product is not compromised. In one study, PRT treatment did not affect the mean 1-hour Corrected Count Increment, representing an adequate response to transfusion. ¹⁴ In addition, Amato, Schennach, Astl et. al.,¹⁵ demonstrated that among patient subpopulations, there were no differences in the platelet concentrate use per patient, number of platelet concentrate units transfused, or amount of time between platelet transfusions between the control and test patients.

As previously stated, bacterial culture and rapid bacterial detection methods do not detect viral or protozoan contamination. In a world of new and emerging pathogens such as Chikungunya Virus, Babesia, Dengue Virus,¹⁶ and most recently, Zika Virus,¹⁷ rapid bacterial detection is at a disadvantage to PRT. Since PRT interferes with nucleic acids, even emerging pathogens (for which the FDA may not have a detection test) will be rendered inactive. This also eliminates the need for cytomegalovirus (CMV) testing.¹⁸

In addition to demonstrating clinical effectiveness of PRT, the PREPAReS trial¹⁹ describes several additional benefits of PRT. Not only did residual white blood cells not proliferate but they also could not present antigens at a normal level. The result was a decreased incidence of Graft-vs-Host disease, and decreased potential HLA alloimmunization. Chi, Zhi, and Vostal²⁰ even demonstrated a reduction in acute lung injury.