Blood Substitutes. Roy Wang Ellen Quach November 2005

Transcription

1 Blood Substitutes Roy Wang Ellen Quach November 2005

2 Why Do We Need Blood Substitutes? Provides disease-free alternative to transfusions Universal Compatibility Longer shelf-life than allogeneic blood Current shortage in donors/eligible donors Religious/Social implications Military/Trauma applications Other applications requiring oxygen delivery (such as organ delivery) Less costly 2

3 What Is In Blood? Stock Photo DennisKunkel.com Plasma Proteins Water Other solutes Formed Elements White Blood Cells Platelets Red Blood Cells 3

4 Red Blood Cells Hematocrit = % of total blood volume occupied by RBCs (determined by centrifuge) Glycolipids in membrane account for ABO & +/- groups Mature RBCs lack organelles/nucleus Anaerobic (due to lack of mitochondria) therefore does not use up O 2 Primarily contains hemoglobin (Hb) 4

5 Physical Properties of Blood Viscosity, non-newtonian, dependant on hematocrit (Hct), Temperature (T), shear rate (γ) μ(γ,hct, T) = (e λ/γ + ηhct )[α + φ/(1+e β(τ-ε) )] Blood Flow : Q = Pr 4 /8μl Density Oxygen dissociation Non-Newtonian: τ = K(δu/δy) n Pseudo-plastic or shear-thinning, thus n<1 5

6 Heart Beat Blood Pressure = Cardiac Output * Total Peripheral Resistance Sensory tissues on the surface of blood vessels. Example: higher viscosity in blood would force the pressure to go up, then the heart would work harder. CO = heart rate * stroke volume TPR = 8µl/r 4 pi Vasoconstriction Hypertension. Nitric oxide help dilate vessels 6

7 Pulsatile Flow and Blood Pressure Reference (10) Reference (13) 7

8 Arteries Elastic arteries are the largest arteries and receive the main output of the left ventricle The elastic tissue in their walls provides the resilience to smooth out the pressure wave Muscular arteries have a media composed almost entirely of smooth muscle 8

9 Arteriolar Vessels Small vessel radius compared to arteries High resistance area results in large pressure drops, smoothing out the pulsatile blood flow from the heart Large amounts of smooth muscles compared to tube radius Arteriovenous shunts bypass capilliaries 9

10 Capillaries Major site of matter exchange Microvasculature Tiny radius on the scale of µm Short distance to tissue Slow & steady blood flow Large surface area (~700 times that of aorta) Drains into a series of vessels of increasing diameter to form venules and veins Oedema occurs when pressure is high 10

11 Non-Newtonian Blood Blood flow in capillaries smaller than ~1mm Viscosity depends on vessel radius Plasma Skimming: Decrease in blood vessel size induces decrease in blood viscosity due to the formation of cell free zone near cell wall Cell free zone between 1µm -5µm in blood with a hematocrit between 40% ~ 50% 11

12 Cell Deformation Red blood cell have a biconcave shape and an average diameter of about 8 µm. To pass through the very narrow capillaries they undergo elastic deformation. 12

13 Hemoglobin Most blood substitutes refer to Hb substitutes derived from RBCs Hb in the RBC is composed of two pairs of protein subunits which merge to produce a tetramer β α α β -Hb undergoes a conformational change when it releases O 2 and picks up CO 2 13

14 Oxyhemoglobin Dissociation Curve Right shift: lower affinity for O 2 Left shift: larger affinity for O2 P 50 is the partial pressure of oxygen in the blood at which the hemoglobin is 50% saturated, typically about 26.6 mmhg 14

15 Shift Factors Increase Decrease P(CO 2 ) Shifts right Shifts right ph Shifts left Shifts right Temperature Shifts right Shifts left CO Shifts right Shifts left 2,3-DPG Shifts right Shifts left Increase in CO 2 just reacts with water to get H + CO 2 + H 2 O < > H + + HCO -3 15

16 Perfluorocarbons Chemically inert, hydrophobic Currently, Oxygent is the most advanced PFC Alliance claims it provides immediate O 2 delivery Phase 3 general surgery study in Europe: patients who received Oxygent either completely avoided the need for blood more frequently than the control group, or required fewer units of blood than the controls. (3) Found avoidance and reduction of blood use in 3 weeks of post-op Intravenous oxygenates in lungs, delivers O2, picks up CO2, expirates in lung Diffusive absorbs/releases through simple diffusion, unlike Hb Small 0.2μ unlike RBCs 7.0μ No binding of NO, CO (no vasoconstriction) Lecithin (phospholipid) (Image Alliance Pharmaceutical Corp.) Adjustable viscosity 2 year shelf life, several days half-life 16

17 Perfluorocarbons 17 Reference (4)

18 Extraction Of Hb From RBCs Centrifuge blood Remove WBCs/platelets and plasma by aspiration Wash RBCs Centrifuge again Lower layer = Hb Extract stroma-lipid and cellular debris through aspiration Dialyze Hb in Ringer s Lactate Solution Plasma & Solutes - yellowish WBCs/Platelets - buffy layer RBCs -deep crimson 18

19 The Beginnings of Hb Substitutes Lysed RBC membrane stroma is toxic to kidney Animal testing of stroma-free Hb is effective and successful Human testing produced renal and cardiovascular problems Found that Hb is toxic outside of the RBCs, breaks into 2 dimers which cannot be metabolized by the kidneys 19

20 Initial Modifications See Reference 20(1)

21 Clinical Trial Phases for Major Types Table was compiled in July of 2004 (5) 21

22 Encapsulated and X-linked Hb First Generation Blood Substitute Solved dimer problem by intramolecular crosslinkage of dimers and by artificial phospholipid bilayer keeping Hb inside a vesicle Small size meant able to bypass blockages to reach smaller vessels High P 50 meant easy release of O 2 22

23 Issues with 1 st Generation Small molecules can bypass blockages but: Thins blood, reducing shear stress in capillaries and resulting in vasoconstriction Molecules are now able to diffuse through blood vessel, mopping up NO Low oxygen affinity means O 2 is easily diffused but: O 2 is too easily let go, diffuses in arteries instead of capillary beds! Vascular Wall 23

24 Polymerized Hb Creates chains of hemoglobin Get larger molecules that won t mop up NO Increases shear stress Still have low oxygen affinity Gives up oxygen more easily Two major brands in clinical phase III PolyHeme (Northfield Labs, over 1 year shelf life) Clinical Trials in Trauma being conducted through exception in informed consent requirements in the U.S. (21 CFR ) (Ethical Implications) Hemopure (Biopure) Study in 2000 found adverse effects, have since been rectified and are continuing in clinical testing 24

25 Conjugated Hb Reference (9) MP4 (Hemospan by Sangart) Six strands of PEG are attached to Hb Larger molecular radius Higher viscosity Lower diffusion coefficient More similar to blood Lower P 50 Won t let go of O 2 until capillary beds, sends O 2 to most deprived tissue Phase II trial found no adverse affects (including those normally associated with blood substitutes) 25

26 Conjugated Vs Polymerized 26

27 Blood Pressure Effects Hypertension occurs after 30 minutes in both crosslinked and polymerized forms (8) 27

28 Oxygen Dissociation O2 release in encapsulated about equal in both O2 release in poly is greater in arterioles O2 release in MP4 is greater in capillary beds (8) 28

29 Recombinant Hb (3 rd Generation) New method of producing hemoglobin Using e.coli bacteria to proliferate hemoglobin No use of human or bovine blood components at all Potentially limitless supply of blood replacement for trauma and surgery Genetic Engineering 29

30 References 1) Chang, Thomas M.S. Artificial Cells & Organs Research Centre McGill University Retrieved Sept 26, ) Chang, Thomas M.S.. Blood Substitutes: Principles, Methods, Products and Clinical Trials Tissue Engineering: Volume I. Karger Landes Systems. Montreal, QC ) Oxygent, Alliance Pharmaceutical Corp. Retrieved Oct 30, ) Riess, Jean G. Understanding the Fundamentals of Perfluorocarbons and Perfluorocarbon Emulsions Relevant to In Vivo Oxygen Delivery Artificial Cells, Blood Substitutes, and Biotechnology, 33: 47 63, ) The Search for a Blood Substitute, United Blood Services: Blood News. July 2004 Edition 6) Wang, Linda. Blood Relatives Science News. March 31, 2001 Volume 159, p206. Retrieved Sept 29, 2005 on InfoTrac 7) Westphal, S.P. Breaking the Rules on Artificial Blood., The New Scientist. March 13, 2004 Volume 181, p8. Retrieved Sept 29, 2005 on InfoTrac 8) Winslow, Robert M. Targeted O2 Delivery by Low-p50 Hemoglobin: A New Basis for Hemoglobin-based Oxygen Carriers, Artificial Cells, Blood Substitutes, and Biotechnology, 33: 1 12, ) Björkholm, M, Fagrell, B. et al. A phase I single blind clinical trial of a new oxygen transport agent (MP4), human hemoglobin modified with maleimide-activated polyethylene glycol Haematologica. Issue 90, Volume 4, ) Blood Pressure, John Ross. Retrieved Oct 30, ) Gas Exchange, Blood Gases Manual. Retrieved Oct 30, ) The Search for a Blood Substitute, United Blood Services: Blood News. July 2004 Edition. 13) S. A. Berger, E. W. Goldsmith, and E. R. Lewis, "Introduction to Bioengineering," 30 Oxford University Press, 2000