Hemoglobin-based Oxygen Carriers

SEMISYNTHETIC or synthetic preparations of hemoglobin, now termed hemoglobin-based oxygen carriers (HBOCs), have been in development as an alternative to erythrocytes for several decades. Nonclinical and clinical studies of HBOCs have raised questions about their safety. Similarity of the serious adverse event profiles among these products has raised questions regarding the possibility of common underlying mechanism(s) of toxicity despite differences among these molecular preparations. The observed serious adverse events have presented an obstacle to development because they raise concerns about the relative benefit/risk of these biologics.A conference sponsored by the Food and Drug Administration (FDA) and National Institutes of Health (NIH) was held April 29–30, 2008 to (1) review the existing publicly available information about the characteristics and clinical profiles of the products that are or were in development; (2) discuss the potential mechanisms of toxicity both for the whole organism or by specific organ system(s); and (3) consider the feasibility of, obstacles to, and ethical issues related to future clinical trials of HBOCs.This report summarizes the scientific presentations and opinions of the speakers at the conference. A transcript of the meeting is available.‡§Appendix 1contains a list of the members of the Planning Committee. Appendix 2contains a list of the Speakers. Appendix 3summarizes information about conflict of interest available to the FDA at the time of the workshop.The workshop was divided into four sessions. Session I provided a forum for a scientific overview of oxygen physiology, general biochemical and physiologic characteristics of HBOCs, nitric oxide physiology, and characteristics of nonclinical studies of HBOCs. Presentations in Session II provided an overview of current information about the safety and efficacy of HBOCs that have been evaluated in clinical trials, the regulatory and ethical framework for clinical trials of HBOCs, and unresolved issues related to publicly available information from commercially sponsored studies of HBOCs. FDA and industry representatives discussed data from clinical trials of HBOCs that are publicly available from peer-reviewed literature, press releases, or presentations at public meetings. Session III was divided into two parts. In the first part, functional aspects of HBOCs as a class of therapeutic agents were discussed, with particular emphasis on common clinical findings, need for additional research into mechanisms of toxicity, clinical trial design issues, and clinical settings where benefit might outweigh safety issues. In the second portion, organ-specific aspects of HBOC safety were addressed. Panelists discussed possible mechanisms for observed safety issues and the pros and cons of sensitive and specific biomarkers for evaluation of organ-specific toxicity. Session IV was devoted to seeking a way forward in development of HBOCs. Presentations were made on new modifications of HBOCs, effects of HBOCs on the microvasculature, molecular biologic reengineering of HBOCs to achieve desired physiologic effects, development of preclinical models, and focused clinical trial designs were discussed. Panelists then discussed ethics, the feasibility of designing new molecules, various clinical trial designs, and the need for HBOCs in civilian and military settings and in ex-U.S. sites.Moderator: Joseph Fratantoni, M.D., Vice President Medical and Clinical Development, Maxcyte, Inc., Gaithersburg, Maryland.Speakers noted that regulation of oxygen transport/oxygen physiology, including physiologically appropriate oxygen affinity and cooperativity, and hypoxic vasodilatation appear to be fundamental to appreciating the complexities of HBOCs and to understanding how a patient’s organ and whole body physiology adjust to hypoxia. Hypotheses with varying degrees of experimental validation have been published to explain the phenomenon of hypoxic vasodilatation including (1) release of adenosine triphosphate (ATP) and binding of ATP to an endothelial cell receptor, resulting in increased circulating nitric oxide and vasodilatation, (2) allosteric release of nitric oxide by sulfhydryl-linked nitric oxide that occurs as the red blood cell (RBC) deoxygenates,1(3) partially oxygen-saturated hemoglobin acting as a nitrite reductase,2and (4) metabolic autoregulation of blood flow.Nitric oxide, a major systemic vasodilator, is a short-lived molecule that is produced by, among other cell types, endothelial cells, and it is released and diffuses both luminally and abluminally. The importance of nitric oxide in modulating downstream effects of acellular hemoglobin was discussed. Some investigators have challenged long-held assumptions that nitric oxide scavenging by HBOC results in impaired blood flow and that oxygen binding of HBOC should match that of RBC.3,4They hypothesize that the presence of low-oxygen-affinity hemoglobin intraluminally disrupts the oxygen gradient from red blood cell to vessel wall and leads to vasoconstriction. Other researchers have emphasized the extent to which the heme moiety in acellular hemoglobin is subject to oxidation. of hemoglobin the of the red blood cell is by of the red blood hemoglobin and is subject to the effects of agents as and the effects of with nitric modifications to the hemoglobin to and in to and to of the have been a of the HBOC and safety and toxicity of the various HBOCs in preclinical have been to the adverse observed in clinical of HBOCs to has been in two (1) safety in (2) or efficacy settings that are and on effects observed in studies in of clinical of HBOCs, of which have been in with with are A of these and is the presence of endothelial including impaired nitric oxide and of an HBOC into the endothelial by of the impaired nitric oxide of toxicity studies feasibility and of and to and to possible of HBOCs with M.D., National for National Institutes of unresolved issues that have the development of HBOCs have in clinical benefit and and efficacy understanding safety that a of products be and an for clinical in studies where and in studies where be 1contains publicly available information about the characteristics of commercially sponsored HBOCs and a of clinical in which these HBOCs were available information press releases, meeting on the safety of HBOC products in a of clinical settings was the data were available in the public for The safety data the for the regarding these data are in the existing clinical trial data into for the of and was in the framework of general ethical and two FDA of of an and of an in is possible The the need for research it is to the ethical framework by and are a of and that be for clinical research to be the need first for the research to have and and scientific and to be where subject is Other the need for a for potential and and review of the The for of benefit to the in a trial with from is of the preclinical and clinical studies in other settings and other to clinical The in the that be an that in with 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events were the of a of clinical and with with and on the and with and on the the of to the HBOC red cell their hemoglobin was that of the red cells, which might have both efficacy and safety that was the in the of serious adverse presented preclinical studies organ and blood flow and oxygen in to to that was increased in of in and presented studies in and a in to that blood findings, and were by of and as by was with of into the data were that have been issues regarding of or about and other adverse adverse events in in that the adverse events observed in clinical trials of with red blood in clinical trials red blood are that should be evaluated in trials where blood is or an a hemoglobin and hemoglobin products in or whole on by presented a of the development of on and the of clinical preclinical of several safety including the development of in effects, and In the were as or to by and of in the and The which with a and a with studies in that an of of the was to The were with in and were functional in with the including on the development of the on with the molecules, of the hemoglobin and of the of nitric oxide scavenging The of these to is was as an in systemic blood that was with hemoglobin and nitric oxide scavenging and to and at two trials the of in the of trial was in the with of in of The second trial was an evaluation of in to of including red blood The trial was because of in the The was with a for for was noted as safety for blood and among in the data from preclinical studies in that of of with of have an adverse first by and by clinical and to the of and effects, a second was with with nitric to in studies in development of that HBOC have that are to and including oxygen and and that these might have a hemoglobin that is with as a nitric oxide for the of oxide is to be the common in nitric oxide of and with and of a of of hemoglobin in the of with systemic were and with of and to first of among was in in at and were among for the in on was for for on the was for the two for hemoglobin and for was a in and from and hemoglobin, an in the need for for or review and the of events by investigators were to that a review of events be in future and that the should be at of to to of as or the and of of at the of the provided a of the noted the (1) of an adverse safety in a clinical trial to an of be data to a major at a or (2) be the of the on be that the of benefit that be clinical of the with the be (3) to the or the for is an of for trials because a in to the the and the experimental of the (4) and be with an is the of into the clinical of III was divided into two sessions. The first was devoted to of clinical and mechanisms of toxicity. The second was devoted to of organ-specific aspects of safety members were to The with and M.D., National Institutes of M.D., four questions for the (1) information about the safety and efficacy from clinical trials in be to (2) is about the and of the current and HBOCs, safety information from the of HBOC be to safety and for a (3) or these and a patient’s underlying that are common to of these (4) from was in II for designing future in of and in and were discussed by M.D., of of of and of noted that results in the in for with blood is the in with and blood is and into two clinical studies because of the in of with a blood and HBOCs on a were M.D., Medical discussed in the design of clinical trials to the current of HBOCs for noted that preclinical data should HBOC clinical trial design for and studies in preclinical of with or and that was a potential benefit of HBOC from to in of design and of a with and and should clinical evaluation of the current of HBOCs with of Medical as an of increased and discussed an physiologic of blood and that has for among with four noted that were at particular of and presented data that the and to red blood cell to and from to at results were despite of of noted has been to of oxygen with hemoglobin, the and as of Health discussed the potential of HBOCs blood is available in both and settings and the need to consider the of and of an HBOC in these M.D., of and The of that HBOCs a need blood is available or is National Institutes of presented the results of of publicly trials of HBOCs and trials in several clinical HBOC products have as common the to nitric oxide and to with nitric oxide that HBOC products are to in A for of the results for these trials was for or and data were by a was a in the of and of the studies that the increased was to a particular a particular or a particular on the of that HBOCs are with a increased of and and that clinical trials of these products should the underlying mechanisms of toxicity have been and and Inc., presented information about the and effects of of the noted that blood in are with in blood flow to are related to the presence of circulating in experimental to is by M.D., of of at discussed the two trials of in on and the of on to HBOCs and on effects of on blood and were as (1) time in on and the of was (2) of blood among (3) and data differences on and (4) as blood of at and possible the of to of with presented with a of increased the by that with of should be from with a from to have a and of clinical trial on might be to with an and of Medical of presented data from a trial of red blood of to and of of a of of red blood discussed the of HBOCs, hemoglobin nitric oxide and HBOC preparations systemic and nitric oxide binding and increased have been to metabolic oxide binding might be of therapeutic benefit in clinical settings as and to a blood might to of raised the of the of HBOC on because have been an in clinical the and were about clinical despite the safety with the current of HBOCs, clinical trials might be was a of opinions on from trials in with to to of the need to to of in Panelists that in of the current safety of it be to HBOC products red blood in an of members that research to be to the effects of nitric oxide scavenging by HBOCs, with for additional research in of and clinical nitric has in and adverse effects on the of Some members that HBOCs should be noted that the by questions that A of that the relative safety and general of red blood for an for be for red blood cell is an alternative or is a be M.D., of Joseph and discussed toxicity of a specific organ and a of questions and and was with hemoglobin to presence of and of current preparations have made with HBOCs. The of and HBOC trials has been with a in studies by and increased in studies by and in a by of are on to by because of the to the of to the and the general of information about the and resulting in that of are as a of or a of noted that with in that it is to that by that are data from studies to how and and data regarding of HBOCs or red blood effects of heme appear to be oxygen and nitric oxide noted that because it is to HBOC products are with it is to a about of toxicity. is that the HBOCs have a common on related to their to oxygen and related to the of of HBOCs for which public information is available have been with adverse events and at HBOCs have been with at biochemical of The have been of of and as the of the of has because for the and of have been extent of in the extent of and the of in the of the of and of The biochemical of which has is on the circulating of the and In an in circulating of is of The of is on and biochemical of has been by increased circulating of and and by the presence of of has been by in circulating of has been a as that the biochemical of and are of the potential of HBOCs to of and the products were in of was that the are related to nitric oxide scavenging by hemoglobin, which is to (1) of the of to increased in the and (2) or impaired to The of and is to be with development of cell and the of and was noted that it is possible that of oxygen in the might in to the to noted that or HBOCs by a of is an that to be a and of biochemical of that nitric oxide in and to the increased to and partially to the HBOC that nitric oxide in of the Inc., April HBOC products are with the development of in sensitive as or are with and in are to observed with of or of a of nitric oxide The are with in has that of hemoglobin that nitric oxide the development of these clinical trials have an increased of in HBOCs. The of be to the effects of HBOCs be to other as oxide the and of nitric oxide was noted that be of among clinical as an of is specific for be increased in with to other additional clinical and to the The was that the of of increased of should be clinical trials with HBOCs, the effects of HBOC products in the was noted that the HBOCs and physiologically the of is of and be the sensitive that nitric oxide be that the of nitric oxide scavenging on and be the of HBOC products and that be related to the effects of HBOCs on and that it was that of a nitric oxide in of the hemoglobin to hemoglobin in the HBOC products appear to be in to release of to the hemoglobin data about in effects are an HBOCs are in the of an as occurs with or particular is that the various in are sensitive to the effects of hemoglobin and that the of evaluation be to Some sensitive of produced by hemoglobin, including and were in the is a acting because toxicity to be by release of heme and their the appropriate time to in is HBOC Clinical data about are the need for preclinical studies at at time clinical evaluation in or is that the current of and preclinical been with and with from of HBOC a specific that be as an for for HBOC with as as with on or from an HBOC clinical trial with that the the of and to assumptions that the of was it has been that of the of the of of is a in are data regarding the effects of HBOCs. and have been as adverse effects of HBOCs. have to the increased of observed with HBOCs, and the of and with to an increased of of other where hemoglobin is in the be is an of in to of with cell has been that oxygen results in of the and that oxygen by HBOCs to the nitric oxide scavenging by the HBOC is for the with of HBOC explain the that occurs with of an HBOC because oxygen is a in the of a nitric oxide in adverse effects in a that should clinical trials with HBOC products that have nitric oxide scavenging with from of the as HBOC including and of hemoglobin appear to be related to the presence of circulating hemoglobin that disrupts the to the in blood and which nitric oxide in with cell is a by hemoglobin of blood flow to a nitric oxide hemoglobin has been to and to the of nitric oxide to on the safety issues related to HBOC is with particular emphasis on nitric M.D., of and of In the possible to HBOC and potential to the of clinical focused on nitrite as an of nitric has presented a that hemoglobin is an nitrite with nitric oxide as it from to blood flow in with nitrite was presented as of the nitrite of hemoglobin in were presented to the effects of HBOC in which was in in endothelial nitric oxide In other with nitric oxide which increased nitrite in blood in the systemic by of hemoglobin results were observed by with was on the of these that nitrite with HBOCs to oxygen data that hemoglobin is a nitrite that nitric oxide at oxygen to vasodilatation and and to safety and efficacy for HBOCs were discussed by of and is possible to hemoglobin affinity for oxygen of nitric oxide to and and of heme produced with nitric oxide scavenging the of blood of of discussed and the possible nitric oxide scavenging by hemoglobin and the of that increased nitric oxide, the scavenging of nitric oxide by that with HBOCs the physiologic effects of their scavenging of nitric The functional of with of both and HBOCs of increased for of Medical focused on the of HBOCs with mechanisms of hemoglobin and the cell receptor, which have been with to and of is to the hemoglobin as in a of modifications of hemoglobin and on appear to the by which systemic and HBOC of molecular of and systemic in and by circulating and is on the molecular of HBOCs that molecular with and The potential for modifications on hemoglobin to binding with blood and to toxicity with HBOCs. of hemoglobin should be into designing and noted that the should be in nonclinical evaluation to emphasis on that physiology, emphasis on that physiologic in evaluation of to specific organ blood flow and should be as of the nonclinical evaluation of organ specific differences in be with HBOCs. Nonclinical is an The design of specific nonclinical for HBOCs be and on the of the of from clinical and an understanding of and speakers questions about of existing clinical data and for future clinical research about the of existing these opinions a of about class of of of discussed the of in understanding safety of a class of and aspects of safety in clinical clinical trials with HBOCs have various designs with various in several clinical and because is a for HBOCs, the of a is the of a to an appropriate of a including that they have of for efficacy and safety. about HBOC of and toxicity is it to data be for a data are available in the public of data to a is in of it is to that is a and that the to the agents that have been that HBOCs should be evaluated in settings where the is as in at of or where blood is an or M.D., Clinical National Institutes of discussed ethical for HBOCs, that they the several for ethical clinical focused on regarding scientific and of an HBOC in the potential need for the of of is an blood and and at the need for HBOCs, the safety and of red cells, including in and military it is to the of future potential for of is an and the that HBOCs to scientific that clinical trials with HBOCs should be to for red cells, for the possibility of for in the that the HBOC was that be a way to a to that the information presented and discussed in the workshop regarding a for and of of and of Medical of that HBOC clinical trials been by a to the trial design and that be by the of that data regarding efficacy are future trial design should consider (1) appropriate (2) (3) of (4) clinical and that these with that of an HBOC to of a design might be appropriate the is red blood that because blood is and in the of clinical trial should be or a major that in a be an appropriate settings be to blood with to blood and a trial in should increased of blood be to safety relative to blood for of with of and major report summarizes the scientific presentations and opinions of the speakers at a conference sponsored by the FDA and on April 29–30, 2008 to (1) review the existing publicly available information about the characteristics and clinical profiles of the products that are or were in (2) discuss the potential mechanisms of toxicity both for the whole organism or by specific organ and (3) consider the feasibility of, obstacles to, and ethical issues related to future clinical trials of effects observed in studies in of clinical of HBOCs, of which have been in with with are A of these and is the presence of endothelial including impaired nitric oxide and of clinical trials and organ-specific aspects of safety were presented and discussed to to to the information about the safety and efficacy from clinical trials in be to (2) is about the and of the current and HBOCs, safety information from the of HBOC be to safety and for a (3) or these and a patient’s underlying that are common to of these (4) from was discussed for designing future of opinions were regarding these speakers that the for the clinical where is a of and for hemoglobin-based oxygen and appropriate for the efficacy and safety of HBOCs in Joseph Fratantoni, M.D., Vice President Medical and Clinical Development, Maxcyte, Inc., Gaithersburg, M.D., National for National Institutes of M.D., National Institutes of M.D., of M.D., of and of information was available to FDA at the time of on April 29–30, of of interest to the research and of with to the efficacy of new of the published The of hemoglobin red might be research with these for and in the of hemoglobin-based current acting as at discuss of hemoglobin an on and the of III clinical in and presentations workshop are of and the or of the of of the and have a and for evaluation of in clinical In has been provided to including at a has been of in of these has to and for of and HBOC for in preclinical and clinical has an to the FDA to in a and trial has or other related to HBOC on an for the of nitrite for on an for a of for nitric oxide and hemoglobin-based blood and of and a scientific of Vice President of Medical in the with as therapeutic agents to of and have or are for in the blood research research I safety review of of interest were from the and