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Anesth Analg 2004;98:20-29
© 2004 International Anesthesia Research Society

CARDIOVASCULAR ANESTHESIA

Myocardial Protection Using Fructose-1,6-Diphosphate During Coronary Artery Bypass Graft Surgery: A Randomized, Placebo-Controlled Clinical Trial

Bernhard J. Riedel, MBChB, MMed, FCA, FAHA*, Janos Gal, MD, PhD*, Gillian Ellis, PhD{dagger}, Paul J. Marangos, PhD{dagger}, Anthony W. Fox, MD, FFPM, FIBiol{dagger}, and David Royston, MBBS, FRCA*

*Department of Anesthesiology, Royal Brompton & Harefield NHS Trust, London, UK and {dagger}Cypros Pharmaceutical Corporation, Carlsbad, California (now incorporated into Questcor Pharmaceuticals, Inc., Hayward, California)

Address correspondence and reprint requests to Bernhard J. Riedel, Division of Anesthesiology and Critical Care - Box 42, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030. Address email to briedel{at}mdanderson.org


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conclusion
 References
 
In vitro and in vivo studies suggest that fructose-1,6-diphosphate (FDP), an intermediary glycolytic pathway metabolite, ameliorates ischemic tissue injury through increased high-energy phosphate levels and may therefore have cardioprotective properties in patients undergoing coronary artery bypass graft (CABG) surgery. We designed a randomized, placebo-controlled, double-blinded, sequential-cohort, dose-ranging safety study to test 5 FDP dosage regimens in patients (n = 120; 60 FDP, 60 control) undergoing CABG surgery. Of these dosage regimens, 3 produced no benefit, 1 produced improved cardiac function, and 1 required adjustment as a result of metabolic acidosis. This suggests that we achieved the intended effect of a dose-ranging study. The expected response was observed in patients treated with 250 mg/kg FDP IV before surgery and 2.5 mM FDP as a cardioplegic additive (n = 15). These patients had lower serum creatine kinase-MB levels 2, 4, and 6 h after reper fusion (P < 0.05), fewer perioperative myocardial infarctions (P < 0.05), and improved postoperative cardiac function, as evidenced by higher left ventricular stroke work index (LVSWI) 6, 12, and 16 h (P < 0.01) and cardiac index (CI) at 12 and 16 h (P < 0.05) after reperfusion. Overall efficacy of FDP was tested across all regimens that included IV FDP (n = 88; 44 FDP, 44 control) using 2 (FDP versus placebo) x 3 (dose size) factorial analyses. Area-under-curve (AUC) analysis demonstrated a significant increase in CI (AUC-16h, P = 0.013) and LVSWI (AUC-16h, P = 0.003) and reduction in CK-MB levels (AUC-16h, P < 0.05) in FDP-treated patients. The internal consistency of this dataset suggests that FDP may provide myocardial protection in CABG surgery and supports previous laboratory and clinical studies of FDP in ischemic heart disease.

IMPLICATIONS: Fructose-1,6-diphosphate (FDP) may increase high-energy phosphate levels under anaerobic conditions and therefore ameliorate ischemic injury. A dose-ranging safety study for FDP was conducted in patients undergoing coronary artery surgery. Preischemic provision of FDP significantly improved cardiac function and reduced perioperative ischemic injury. These myocardial protective effects may improve patient outcome after cardiac surgery.


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conclusion
 References
 
Coronary artery bypass graft (CABG) surgery is associated with increased risk of perioperative myocardial injury and cardiac dysfunction. Research suggests that myocardial injury in the surgical setting is also predictive of worse long-term clinical outcome (1). Perioperative myocardial injury in CABG surgery predominantly reflects an archetypal ischemia-reperfusion injury whose timing can be anticipated. As a result, a number of cytoprotective strategies addressing different aspects of the complex ischemia-reperfusion injury cascade have been tested with varying results (2). The majority of interventions attempt to modulate this cascade by targeting potential deleterious downstream events, such as polymorphonuclear leukocyte or platelet activation, or attempt to activate adenosine receptors or open adenosine triphosphate (ATP)-sensitive potassium channels. A more global approach to preventing ischemia-reperfusion injury would be an attempt to directly increase high-energy phosphate (ATP) production. Glucose-insulin-potassium (GIK) infusion is one such strategy. In fact, GIK as a supplement to intracoronary thrombolysis has significantly improved in-hospital and long-term follow-up survival after acute myocardial infarction (3,4).

Fructose-1,6-diphosphate (FDP), an endogenous high-energy glycolytic pathway intermediary, also enhances ATP production (Fig. 1). FDP may, however, have several theoretical advantages over GIK. These include independence of insulin action, glycolytic pathway entrance distal to the rate-limiting enzyme phosphofructokinase, and no need for phosphorylation (and hence ATP consumption) before its catabolism. Evidence supporting a role for FDP in alleviating ischemia-reperfusion injury includes increased high-energy phosphate production in several tissues, including ischemic/reperfused myocardium (5–9), and attenuation of intracellular calcium influx (10–12), neutrophil adhesion to endothelium (13), platelet activation (14), and free-radical production (15). To this effect, numerous in vitro and in vivo studies have demonstrated that FDP ameliorates ischemic injury in myocardium (6–9,16–18), brain (19,20), kidney (21), and intestine (22).



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  		Figure 1. Schematic presentation of mammalian glycolysis. Myocyte glucose uptake is predominantly facilitated by insulin-sensitive glucose transporter, GLUT-4 (40). Recent evidence suggests that fructose-1,6-diphosphate uptake may be facilitated by a dicarboxylate transporter (32) that is independent of insulin action. Initial phosphorylation of intracellular glucose results in ATP consumption and therefore a lower yield (2 ATP) compared with that by exogenous fructose-1,6-diphosphate (4 ATP), which is already phosphorylated and enters distal to product inhibition by lactate on phosphofructokinase (PFK). (-), inhibition; NAD, nicotinamide adenine dinucleotide; NADH, reduced form of NAD; PFK, phosphofructokinase; TCA, tricarboxylic acid cycle.

 
The reduction in myocyte ischemic injury translates into improved hemodynamic function in isolated rat (7,17,18), isolated rabbit (8,9,23), and in situ dog (6,16,24) hearts subjected to either ischemia (6,24), ischemia and reperfusion (8,9,18,25), or anoxia and reoxygenation (7,17). These studies are further supported by clinical studies demonstrating that FDP improved hemodynamic function in patients with acute myocardial infarction (26) and chronic ischemic heart disease (27–29). However, the potential beneficial effects of FDP have not been evaluated in patients undergoing cardiac surgery.

We hypothesized that FDP administration might prevent the deleterious effects of an expected period of ischemia with subsequent reperfusion, thereby improving myocardial performance and patient outcome after CABG surgery. Our goal was to establish tolerability and possible efficacy of FDP through a dose-ranging safety study. This article describes our experience with FDP in patients undergoing CABG surgery.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conclusion
 References
 
This study complied with the Declaration of Helsinki (as amended) and Good Clinical Practice Guidelines. After IRB (Hillingdon Health Authority, Middlesex, UK) approval, written informed consent was obtained from patients scheduled for elective CABG surgery using a consent form that complied with Investigational New Drug and Clinical Trial Exemption regulations and guidelines as set out by the Food and Drug Administration (USA) and Medicines Control Agency (UK), respectively. One-hundred-twenty patients with >=2 operable coronary artery lesions and preserved ventricular function (left ventricular [LV] ejection fraction >45%) were studied. Exclusion criteria included preoperative inotropic therapy, previous CABG surgery, concurrent valvular or carotid surgery, insulin-dependent diabetes mellitus, and fructose intolerance (a metabolic disorder with autosomal recessive inheritance).

CordoxTM (CPC-111, Cypros Pharmaceutical Corporation, Carlsbad, CA) consisted of 10% (100 mg/mL) trisodium FDP octahydrate in sterile, pyrogen-free water in glass vials (50 mL) that was acidified to pH 3.9 with 13.37% HCl (1 M), thereby allowing stability for 3 yr at 4°C. This solution was diluted (1:1) with 5% dextrose and infused as a bolus dose over 30 min. The period of infusion was chosen because of the relatively large volumes that were required (a 70-kg patient requires 350 mL).

A randomized, placebo-controlled, double-blinded, sequential-cohort, dose-ranging study was designed to investigate the safety of FDP in patients undergoing CABG surgery and its cardioprotective properties, if any. In all stages of the study, patients were randomized (1:1) to receive either FDP or placebo treatment, both administered in a blinded fashion. For the IV studies, 5% dextrose, matched for volume, was used as placebo. For the cardioplegic studies, the control group received unsupplemented cardioplegic solution. Before randomization studies, 5 open-label patients were initially studied to ensure tolerability of IV FDP and 5 to ensure tolerability of FDP-enhanced cardioplegic solution.

Dosage and administration regimens were as follows (Fig. 2):



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  		Figure 2. Schematic presentation of dosing regimens. CPB, cardiopulmonary bypass; AXC, aortic cross-clamp.

 
A. 125 mg/kg FDP (n = 10) IV infusion before cardiopulmonary bypass (CPB).
B. 250 mg/kg FDP (n = 10) IV infusion before CPB.
C. 2.5 mM FDP (n = 10) supplemented crystalloid and blood cardioplegic solutions.
D. 250 mg/kg FDP (n = 15) IV infusion before CPB combined with 2.5 mM FDP-supplemented crystalloid cardioplegia.
E. 250 mg/kg FDP (n = 15) IV infusion before CPB, followed by a bolus dose of either (E1) 250 mg/kg FDP [n = 6] or (E2) 125 mg/kg FDP [n = 9] 2 and 6 h after reperfusion (aortic cross-clamp release).

All previously prescribed antianginal medications were continued preoperatively. The same anesthesiologist administered a balanced, opioid-based (intermediate-dose) general anesthetic that consisted of propofol (1 to 2 mg/kg), fentanyl (8 to 10 µg/kg, titrated from induction to sternotomy), and pancuronium bromide (0.15 mg/kg) for induction of anesthesia. Anesthesia was maintained with a nitrous oxide/oxygen/isoflurane mixture until aortic cannulation, after which nitrous oxide was replaced with air. During and after CPB, anesthesia was maintained with isoflurane (0.5–1.0 MAC) and a propofol infusion (3 mg · kg-1 · h-1). The propofol infusion was continued into the intensive care unit (ICU) and maintained until the patient was considered ready for tracheal extubation. All patients received an IV infusion of nitroglycerine (0.3 µg · kg-1 · min-1) from induction of anesthesia until 24 h later.

The perfusion system used a membrane oxygenator (Sorin Laboratories, Mirandola, Italy) and roller pump (Cobe Stockert, Stockert Instruments, Rungis, France) and the circuit primed with Hartmann’s solution (1.5 L) and mannitol 20% (1 mL/kg) to achieve moderate hemodilution (hematocrit ~25%). Nonpulsatile flow and mean arterial blood pressure were targeted at 2.4 L · min-1 · m-2 and 50–70 mm Hg, respectively.

Myocardial preservation was achieved with moderate systemic hypothermia (nasopharyngeal temperature of 32°C to 34°C), cold anterograde hyperkalemic crystalloid (St. Thomas’s I) cardioplegia solution (except for the blood cardioplegia stage, Regimen C), and topical cooling of the myocardium with ice-slush within the pericardial sac. Cardioplegia was repeated at 30-min intervals or sooner if electrical activity was observed during electrocardiogram (ECG) monitoring. In all cases an internal thoracic artery was used as well as additional saphenous vein conduits. After surgery, dopamine was used as the first-line drug if inotropic support was required to maintain a cardiac index (CI) of >1.8 L · min-1 · m-2 (to a total dose rate of 8 µg · kg-1 · min-1) and then supplemented with epinephrine (0.03–0.1 µg · kg-1 · min-1).

Central venous and pulmonary artery catheters were introduced after induction of anesthesia for monitoring of central venous pressure, pulmonary artery wedge pressure (PAWP), pulmonary artery pressure (PAP), and cardiac output obtained by thermodilution. CI, LV stroke work index (LVSWI), and other hemodynamic indices (calculated using standard formulae) were measured at baseline (after induction of anesthesia), after FDP/placebo infusion before CPB, and 1, 2, 4, 6, 12, and 16 h after reperfusion.

Total creatine kinase (CK) and its CK-MB isozyme were measured at baseline (before surgery) and 2, 4, 6, 12, 24, 48, and 72 h after reperfusion. Normal ranges at our institution for total CK are 24–195 IU/L for males and 24–170 IU/L for females and <24 IU/L for CK-MB for both sexes. Two or three independent physicians blinded to the study intervention interpreted consecutive daily 12-lead ECGs. Holter monitoring was used for the first 72 h postoperatively.

Perioperative myocardial infarction (P-MI) was diagnosed using the following criteria: CK-MB activity >50 IU/L for a period more than 12 h, and ECG changes on a standard postoperative 12-lead ECG according to the Minnesota code criteria:

a. Q-wave P-MI: The appearance of new, persistent (>24 h) Q-waves >0.04s in >=2 contiguous leads of the same vascular territory or equivalent R-wave increments (R/S ratio >1 in leads V1 and V2).
b. Non-Q-wave P-MI: The appearance of new, persistent (>24 h) conduction abnormalities or ST-T alterations (ST segment depression or elevation >0.1 mV at 0.08s after the J-point or T-wave inversion) in >=2 contiguous leads of the same vascular territory.

Requirement for cardiovascular support, duration of ventilation, and lengths of ICU and hospital stay were recorded. Cardiovascular support was defined as the requirement for inotropic support (dopamine >3 µg · kg-1 · min-1 or epinephrine) and/or antiarrhythmic therapy (pacing, isoproteranol, or amiodarone). Holter monitors in the first 72- postoperative hours also surveyed for postoperative atrial fibrillation. The incidence of hemodynamically significant (systolic blood pressure [SBP] <90 mm Hg) atrial fibrillation (confirmed by 12-lead ECG) requiring antiarrhythmic therapy (amiodarone) was also compared between groups.

Vital signs, laboratory-based hematological and biochemical profiles, and adverse events were recorded for 72 h after reperfusion. Adverse events were coded using the Coding Symbols for Thesaurus of Adverse Reaction Terms dictionary.

Sample size was calculated to detect a 20% difference in mean variable with {alpha} = 0.05 and a power of 0.8 using a standardized SAS® (SAS Institute, Cary, NC) software program. An independent site monitor reviewed all medical records and verified data source and data accuracy. Data are presented as mean ± SD unless stated otherwise. Statistical comparisons of continuous variables were done using one-way analysis of variance (ANOVA), and Fisher’s exact test (two-sided) was used for the comparison of proportions. For variables with multiple assessment times, areas under the curve (AUC) were calculated from the baseline values using the trapezoidal rule. For AUC measures, individual missing values were interpolated (except for values at the first postinfusion time point) if these were not consecutive. Some AUC measures were calculated using the carry-forward methodology. A P value <0.05 indicated statistical significance, and a P value <0.2 was accepted as suggesting a trend.

To evaluate the overall efficacy of FDP, a 2 x 3 factorial analysis was done for all groups (Regimens A, B, D, and E2) that received preischemic IV FDP treatment irrespective of cardioplegic supplement. Groups excluded from these analyses included those on the failed large-dose postreperfusion FDP regimen (Regimen E1) and those that received FDP only via the cardioplegic route (Regimen C). In the 2 x 3 factorial analyses, one factor was treatment group with 2 levels (FDP versus placebo) and the other factor was dose size with 3 levels (125 mg/kg, 250 mg/kg, or 250 mg/kg plus 125 mg/kg 2 and 6 h after reperfusion). Results are presented with three P values (Table 2). These indicate significance or lack of significance for effect of treatment (FDP versus placebo), effect of dose (Regimens A, B, D, and E2), and an interaction between treatment and dose. Should a significant treatment effect exist with no dose or interaction effect, this would indicate a general effect of FDP. However, significant dose and interaction effects would indicate that the effect of treatment might be a function of dose.


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  		Table 2. Cardiac Index (L · min-1 · m-2): Change From Baseline Value for all Preischemic Intravenous FDP (Regimes A, B, D, E2)- and Placebo-Treated Groups
 

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  		Table 1. Baseline Patient Characteristics for FDP (Regimen D)- and Placebo-Treated Groups
 

   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conclusion
 References
 
Section A reports on the results of individual dosing strategies for FDP in which each cohort is compared to the control patients of that group (1:1 randomization). Results in this section are presented in 3 subsections, reflecting the nature of this study: a dose-ranging safety trial with a design aimed at achieving insufficient, adequate, and excessive dosing. Section B reports on the results in which all dosing groups receiving preischemic IV FDP were combined and compared with all placebo patients from the relevant cohorts in an attempt to analyze for overall drug efficacy. Data are presented as mean ± SD unless stated otherwise.

A. Analysis of Individual Groups
i. Single Dose—IV or Cardioplegic FDP [Regimens A-C].
No significant differences were found between the placebo- and single-dose FDP- (125 mg · kg-1 [Regimen A] or 250 mg · kg-1 [Regimen B] IV FDP or 2.5 mM FDP cardioplegic supplement [Regimen C]-treated groups with regard to preoperative demographics, intraoperative variables, hemodynamic indices, ischemic indices, or requirement for cardiovascular support.

ii. Combined IV and Cardioplegic FDP [Regimen D].
Preoperative Demographics and Intraoperative Variables.
FDP- (n = 15) and placebo- (n = 15) treated patients allocated to this group did not differ with respect to demographics, duration of CPB, or aortic cross-clamping (Table 1).

Hemodynamic Indices.
Baseline hemodynamic indices were similar for the FDP- and placebo-treated patients in this cohort: PAWP, 7.7 ± 4. 5 versus 8.7 ± 3.3 mm Hg; CI, 1.95 ± 0.41 versus 1.99 ± 0.71 L · min-1 · m-2; LVSWI, 33.3 ± 7.9 versus 31.7 ± 10.6 g · m-1 · m-2; and pulmonary vascular resistance indexed (PVRI), 313.2 ± 37.4 versus 326.6 ± 45.2 dynes-1 · cm-5

CI values were 17% higher (P = 0.042 for absolute values and P = 0.018 for change from baseline values by ANOVA) 12–24 h postreperfusion in FDP- compared with placebo-treated patients. LVSWI values remained less than baseline (P < 0.05) after reperfusion in the placebo-treated group throughout the study period. In contrast, LVSWI recovered in the FDP-treated group 6 h (P = 0.001) after reperfusion and remained so (Fig. 3). Improved hemodynamic indices in FDP-treated patients could not be attributed to increased preload because PAWP values after reperfusion tended to be slightly higher in placebo-treated patients, but this difference did not achieve statistical significance.



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  		Figure 3. Hemodynamic indices for fructose-1,6-diphosphate (FDP) (Regimen D)- and placebo-treated groups. Mean (± SD) values for left ventricular stroke work index (LVSWI; g · m-1 · m-2; closed symbols) and pulmonary artery wedge pressure (PAWP; mm Hg; open symbols) for FDP- (n = 15) and placebo- (n = 15) treated patients. Increased LVSWI in the FDP-treated group was not associated with raised PAWP. Base, baseline measurement; post-inf., immediate postinfusion period, before commencement of cardiopulmonary bypass; 1–16-h, hours after myocardial reperfusion; *P < 0.05 for comparison between FDP- and placebo-treated groups.

 
Systemic vascular resistance remained similar between FDP- and placebo-treated groups. However, differences in PVRI were observed after reperfusion. During administration of anesthesia and before CPB, PVRI was reduced in both groups. Immediately after reperfusion, an increase above baseline was observed in the placebo-treated group but not in the FDP-treated group. This difference was significant 0 h (228.5 ± 26.5 versus 318.7 ± 30.7 dyne · s-1 · cm-5; P = 0.032) and 2 h after reperfusion (297.1 ± 19.3 versus 409.2 ± 27.2 dyne · s-1 · cm-1; P = 0.002). Thereafter, the PVRI values in the placebo-treated group returned to baseline. PVRI values in the FDP-treated group, however, were significantly reduced 6 (263.9 ± 24.0 versus 337.5 ± 28.4 dyne · s-1 · cm-5; P = 0.047), 12 (239.3 ± 41.3 versus 337.5 ± 30.9 dyne · s-1 · cm-5; P = 0.06 [trend]), and 16 (180.3 ± 31.2 versus 311.0 ± 28.9 dyne · s-1 · cm-5; P = 0.006) h after reperfusion compared with the placebo-treated group.

Analysis of the effect of CPB duration (more or less than 90 min) demonstrated an independent effect on PVRI. Subsequent {chi}2 analysis of AUC data (using placebo-treated patient values as expected data) confirmed this reduction in PVRI (AUC-12h; P = 0.017) with FDP treatment.

Ischemic Indices.
No significant differences were observed for total serum CK levels between FDP- and placebo-treated patients at any of the measured time points.

Serum CK-MB isozyme levels, however, were significantly lower 2 (P = 0.026), 4 (P = 0.017), and 6 h (P = 0.050) after reperfusion in the FDP-treated group (Fig. 4).



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  		Figure 4. Creatine kinase isozyme (CK-MB) values for fructose-1,6-diphosphate (FDP) (Regimen D)- and placebo-treated groups. Preoperative (baseline) and post-reperfusion CK-MB values (mean ± SD) are shown for the FDP- (white bars, n = 15) and placebo-treated (dark bars, n = 15) groups. *P < 0.05 for comparison between FDP- and placebo-treated groups.

 
No placebo-treated patients and one FDP-treated patient suffered a new Q-wave (transmural) P-MI. This was associated with a surgical complication, requiring pledget sutures to ensure hemostasis from a bleeding coronary artery branch. Five placebo- and 0 FDP-treated patients suffered a new non-Q-wave (subendocardial) P-MI (P = 0.042).

Cardiovascular Support.
Atrial fibrillation that necessitated treatment (SBP <90 mm Hg) occurred in 1 of 15 FDP- and 5 of 15 placebo-treated patients but failed to reach statistical significance (P = 0.17). Differences in inotropic support requirement (2 of 15 versus 5 of 15; P = 0.39 required dopamine; and 0 of 15 versus 2 of 15 required epinephrine) after weaning from CPB and during the first 16 postoperative hours did not achieve statistical significance between FDP- and placebo-treated groups, respectively.

Patient Management.
No significant differences were noted between FDP- and placebo-treated patients with regards to duration of mechanical ventilation (10.1 ± 4.9 h versus 23.0 ± 28.8 h; P = 0.096) and duration of ICU stay (20.33 ± 3.50 h versus 37.02 ± 32.75 h; P = 0.06), respectively.

iii. Combined Preischemic and Reperfusion IV FDP (Regimen E).
Regimen E1
Metabolic acidosis (base excess [BE] >-6, normal PCO2) requiring treatment with sodium bicarbonate was observed in 5 of 6 patients who received the large-dose postreperfusion FDP regimen (Regimen E1). Mean BE values 2 h after reperfusion were -6.5 ± 2.0 versus -3.3 ± 0.79, P < 0.05, for the FDP- and placebo-treated groups, respectively. This acidosis was accompanied by a nonsignificant reduction in CI and LVSWI and postoperative atrial fibrillation in 4 of 6 patients. Despite the development of metabolic acidosis, lactate levels were not increased (data not shown). As a result, the reperfusion doses were reduced by 50% to 125 mg/kg for the remaining 9 patients (Regimen E2).

Regimen E2
Significantly higher CI (3.17 ± 0.55 versus 2.63 ± 0.36 L · min-1 · m-2; P = 0.032, and 3.08 ± 0.68 versus 2.31 ± 0.51 L · min-1 · m-2; P = 0.017) values were observed 12 and 16 h after reperfusion in FDP- compared with placebo-treated patients, respectively. However, significant differences for LVSWI (35.22 ± 6.33 versus 29.88 ± 4.35 g · m-1 · m-2; P = 0.064, and 36.11 ± 7.68 versus 29.44 ± 7.28 g · m-1 · m-2; P = 0.077 at 12 and 16 h after reperfusion, respectively) were not observed in FDP- compared to placebo-treated patients. FDP-treated patients, however, continued to display a susceptibility to metabolic acidosis (3 of 9 versus 1 of 9 in the placebo group; P = 0.576) and postoperative atrial fibrillation (5 of 9 versus 1 of 9 in the placebo group; P = 0.131) with this postreperfusion regimen.

B. Analysis of all IV Regimens for Overall Drug Efficacy
Factorial analysis was done for all groups (Regimens A, B, D, and E2) that received IV FDP to determine the overall efficacy of FDP treatment. Data from patients who received the failed large-dose postreperfusion FDP regimen (Regimen E1) and those who received FDP only via the cardioplegic route (Regimen C) were excluded from this factorial analysis.

Hemodynamic Indices.
Baseline CI values were similar for all FDP- (1.86 ± 0.37 L · min-1 · m-2, n = 44) and all placebo- (2.00 ± 0.57 L · min-1 · m-2, n = 43) treated groups. However, a larger change from baseline values for CI was observed immediately after FDP infusion and 4, 12, and 16 h after reperfusion (Table 2) compared with the placebo group. AUC analysis demonstrated a significant increase in CI (AUC-6h, P = 0.032; AUC-12h, P = 0.012; AUC-16h, P = 0.013) and LVSWI (AUC-16h, P = 0.003) in the FDP-treated patient groups, and is consistent with a general treatment effect of FDP.

Ischemic Indices: CK-MB Release.
Factorial analysis showed an interaction effect with decreased CK-MB release in FDP-treated groups at 2 (P = 0.039) and 4 (P = 0.016) h after reperfusion. AUC analysis demonstrated a significant reduction in CK-MB (AUC-16h, P = 0.046) levels in FDP-treated groups, and is consistent with a dose-dependent effect of FDP.

C. Overall Tolerability
Studies have reported that FDP may chelate ionized calcium and increase serum phosphate levels (30). Vital signs, laboratory-based hematological and biochemical profiles, and adverse events were recorded for 72 h after reperfusion. Serum calcium and phosphate levels did not differ significantly from baseline values 24, 48, and 72 h after surgery. Preischemic IV and cardioplegic dosing regimens of FDP were well tolerated and no serious drug-related adverse events were reported. Metabolic acidosis requiring treatment with sodium bicarbonate, however, occurred in the large-dose postreperfusion FDP regimen (Regimen E1). This was accompanied by nonsignificant reduction in CI and LVSWI and increased incidence of atrial fibrillation (4 of 6 patients). As a result, dose modification was required (Regimen E2).


   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conclusion
 References
 
This clinical dose-ranging safety study investigated the potential use of pretreatment with FDP to prevent the deleterious effects of an expected period of ischemia with subsequent reperfusion in surgical coronary revascularization. The study was conducted in a relatively low-risk patient population scheduled for elective CABG surgery.

FDP was well tolerated in all preischemic dosage regimens. However, administration in the postreperfusion period resulted in metabolic acidosis. The desired dose-ranging effects of "under" dosing (Regimens A & B), "adequate" dosing (Regimen D), and "excessive" dosing (Regimen E) were also achieved. Having established the safety and a dose-ranging effect of preischemic FDP administration, we tested the overall efficacy of IV FDP across all treatment regimens. These data demonstrated a dose-dependent myocardial protective treatment effect, which was maintained across all the study groups despite expected "dilution" of treatment effects by the suboptimal dosing regimens. The effects support previous studies reporting beneficial effects of FDP in ischemic myocardium in animals (6,8,16,18,23,27), and after acute myocardial infarction (26) and established heart failure (27–29) in humans. It is interesting to note that FDP improved cardiac (LV) function most in those patients with impaired LV function and increased LV filling pressures (26–29).

Beneficial treatment effects with myocardial protection were best noted in preischemic dosing Regimen D. No added benefit was obtained from additional postischemic dosing (Regimen E). This suggests that Regimen D achieved a ceiling effect. Administration of FDP late in the postreperfusion period, after presumably successful revascularization, did not provide additional benefit. As numerous in vivo and in vitro studies demonstrate, FDP increases high-energy phosphate (ATP) production in several tissues, including ischemic/reperfused myocardium (5–9), suggesting that membrane-associated ion pumps and channels are better maintained during ischemia and reperfusion.

Adequate preischemic dosing was associated with lessened ischemia-reperfusion injury, as judged by accelerated return to baseline of hemodynamic indices (CI, LVSWI), fewer non-Q-wave perioperative myocardial infarctions, and indirect indices such as reduced cardiac enzyme release. In addition, protective effects against ischemia-reperfusion injury of the pulmonary vascular bed may explain the observed reduction in postoperative pulmonary vasoconstriction.

Ventricular function, depicted by the relationship between preload (PAWP) and ventricular work (LVSWI), the Frank-Starling mechanism, demonstrated accelerated recovery to preischemic levels in the FDP-treated group, with a resulting reduction in PAWP and accompanying increase in LVSWI (Fig. 3). In contrast, ventricular function failed to recover to baseline level within the first 16 postoperative hours in the placebo-treated group. This decreased inotropic state was most likely consequent to ischemia-reperfusion injury, e.g., myocardial stunning or infarction. Importantly, the rapid hemodynamic recovery associated with FDP pretreatment was not related to an increase in preload and therefore seems consistent with a treatment effect of FDP that is intrinsic to the myocardium. These dose-dependent myocardial protective treatment effects were maintained across all the study groups receiving IV FDP, despite the fact that not all groups had FDP-supplemented cardioplegia. The potential role for IV FDP in providing metabolic support to the "unprotected heart" during beating-heart off-pump coronary artery bypass surgery where obligatory ischemia still occurs therefore seems warranted.

Pulmonary vasoconstriction is a known consequence of ischemia-reperfusion injury and often follows CPB. The relationship observed between prolonged duration of CPB and increased PVRI is consistent with this notion. The absence of pulmonary vasoconstriction after IV administered FDP (compared with increased PVRI 0–2 hours after reperfusion in the placebo-treated group) suggests that FDP may protect the pulmonary vascular bed during CPB.

Alleviation of ischemia-reperfusion injury through increased glycolytic ATP generation may, however, not fully explain the late-onset pulmonary vasodilation (reduction in PVRI 6–16 hours after reperfusion) observed in the FDP-pretreated group or the observed reduction in PAP by FDP in other studies (29,30). We suggest that this temporal component of active pulmonary vasodilation by FDP may involve upregulation of an intracellular pathway.

As FDP is hydrophilic, it is understandable that questions arise regarding its ability to access cellular membranes for glycolytic ATP generation or an intracellular effect, as postulated above, to occur. However, cellular accession by FDP but not by fructose, fructose-1-phosphate, or fructose-6-phosphate has been demonstrated by spectrophotometric studies in artificial membrane bilayers (31). This and evidence that FDP may ride an active transporter (32) support the notion that by crossing membranes the diphosphate itself (rather than its catabolites) may induce the observed therapeutic effects.

Atrial fibrillation remains one of the most common complications of CABG surgery, and is usually associated with increased duration of hospital stay. The reduction in incidence of atrial fibrillation and durations of intensive care and hospital lengths of stay failed to achieve statistical significance with the effective FDP-dosage regimen (Regimen D). Whether increased glycolytic ATP generation by FDP with subsequent reduction in atrial ischemia-reperfusion injury or improved cation homeostasis, as supported by studies demonstrating restored contractility in myocardial tissue depolarized with potassium chloride (33) and protective effects against cardiac glycoside-induced hyperkalemia (34) will reduce postoperative atrial fibrillation remains unanswered.

Although it is not immediately obvious why the incidence of atrial fibrillation increased in the subgroup administered postreperfusion bolus infusions of FDP (Regimen E), the metabolic acidosis that occurred in the postreperfusion dosage regimens may have been a contributing factor. The cause of the metabolic acidosis is not obvious. As reported, lactatemia was not observed and this is consistent with evidence suggesting that acidosis accompanying ischemia follows failure to fuel ionic pumps, as a result of reduced ATP derived from glycolysis rather than from lactic acid dissociation (35), and that intracellular acidosis and lactate levels are dissociated under hypoxic conditions, at least in brain tissue (36).

However, the FDP was buffered in a HCl-containing solution before reconstitution, and hepatic metabolic pathways or endogenous acid-base systems may not have functioned optimally in the reperfusion period and thus been unable to attenuate this acid load. Lyophilized FDP that does not require acidic buffering in solution has recently been developed and may refute or confirm these findings in subsequent studies.

Study Limitations and Future Directions
Major limitations of this dose-ranging safety trial include the use of small patient cohorts, a relatively low-risk study population, and end-points that are surrogates of outcome (e.g., CI, LVSWI, CK-MB). More rigorously designed, adequately powered, prospectively randomized efficacy studies incorporating high-risk patients and hard outcome trial end-points (e.g., death) are required to confirm or refute the beneficial effects of FDP in cardiac surgery. Other bolus-dosing studies that demonstrated benefit only in patients with impaired myocardial function support the inclusion of high-risk patients (26,29). Furthermore, we may be criticized for a frequent incidence of non-Q-wave myocardial infarction (MI) in the placebo group of Regimen D. This may, however, reflect the definition of PMI used in this study. By virtue of conventional on-pump CABG surgery requiring aortic cross-clamping, all CABG patients are subjected to varying degrees of ischemia-reperfusion injury. The yield of clinically identifiable injury will invariably reflect the sensitivity of the assay system and the definition used. The frequent incidence of non-Q-wave MI observed in our study correlates with that observed by other investigators. Using sensitive biomarkers such as technetium pyrophosphate scanning (37) and cardiac troponin isozyme assays (38), others have reported incidences of non-Q-wave MI as frequent as 30% after routine CABG surgery. In fact, one multicenter study revealed a 25% overall incidence of Q-wave MI after CABG surgery (39). Because recent data suggest that minor ischemic injury is detrimental to long-term patient outcome, we would further argue for more stringent diagnostic criteria when examining myocardial protective strategies and therefore recommend that troponin measurement be implemented in future trials.


   Conclusion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusion
References
 
This dose-ranging safety study conducted in a low-risk patient population scheduled for elective coronary revascularization shows promising dose-dependent cardioprotective effects for FDP. Evidence of improved hemodynamic recovery and reduced ischemic injury were observed when FDP was administered in an effective preischemic dosing strategy. These findings suggest that further adequately powered studies investigating the impact of FDP on perioperative outcome after CABG surgery appear justified.


   Acknowledgments
 
Supported, in part, by Cypros Pharmaceutical Corporation, Carlsbad, California (Protocol FDP-202).

Joan Buffini, BS, helped us with GCP compliance and study administration. Fred Hoehler, PhD, gave us patient statistical support. Eva Procopczuk, MD, PhD, made valuable comments on an earlier draft of this manuscript.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusion
 References
 

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Accepted for publication August 19, 2003.




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Lippincott, Williams & Wilkins Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins and Stanford University Libraries' HighWire Press®. Copyright 2004 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999 HighWire Press