Predicting immunosuppressant dosing in the ... - Wiley Online Library

LIVER TRANSPLANTATION 14:46-52, 2008

ORIGINAL ARTICLE

Predicting Immunosuppressant Dosing in the Early Postoperative Period with Noninvasive Indocyanine Green Elimination Following Orthotopic Liver Transplantation Brian M. Parker,1,2 Jacek B. Cywinski,1 Joan M. Alster,3 Samuel A. Irefin,1,2 Marc Popovich,1 Michael Beven,4 and John J. Fung2 1 Department of General Anesthesiology, 2Transplant Center, 3Quantitative Health Sciences, and 4Center for Anesthesiology Research, Cleveland Clinic, Cleveland, OH

Twenty adult patients undergoing orthotopic liver transplantation (OLT) were enrolled in this study, with the noninvasive indocyanine green plasma disappearance rate (ICG-PDR) measured both during and after OLT to assess the relationship between ICG-PDR and the ability of patients to achieve therapeutic postoperative tacrolimus immunosuppressant blood levels. Liver function was determined at both 2 and 18 hours post reperfusion with the ICG-PDR k value (1/min). Postoperative standard serum measures of liver function as well as liver biopsies were also collected and analyzed. The median ICG-PDR k value for the study group at 2 hours post reperfusion was 0.20 (0.16, 0.27), whereas at 18 hours post reperfusion, it was 0.22 (0.18, 0.35). The median change in the k value between the two ICG-PDR measurements was 0.05 (⫺0.02, 0.07) with P ⫽ 0.02. There was an interaction between the postoperative day 1 (18 hours post reperfusion) ICG-PDR k value and the linear increase in the tacrolimus blood level, such that the greater the k value was, the more gradual the observed rise was in tacrolimus over time [that is, the longer it took to achieve a therapeutic blood level (⬎12 ng/mL), P ⫽ 0.003]. Of the 16 patients that received tacrolimus, comparable dosing on a per kilogram body weight basis was observed. Also, no significant association between ICG-PDR k values and postoperative liver biopsy results was seen. This study demonstrates that the ICG-PDR measurement is a modality with the potential to assist in achieving adequate blood levels of tacrolimus following OLT. Liver Transpl 14:46-52, 2008. © 2007 AASLD. Received April 20, 2007; accepted July 31, 2007.

Postoperative liver allograft function can significantly influence immunosuppressant levels, and as a result, frequent dosing changes of these agents are often required. Common immunosuppressive agents including cyclosporine, tacrolimus, and sirolimus are frequently dose-adjusted, as determined by measurements of whole blood concentration. In addition, these agents are principally metabolized by hepatic cytochrome enzymes, which are variably affected by age, blood flow, ischemia reperfusion (IR) injury, and other factors.1-3 Although this approach is commonly

used in clinical practice, it provides a relatively late prediction of the patient’s response to these drugs and can potentially lead to a prolonged period of suboptimal levels of a given immunosuppressant in the postoperative period, posing an increased risk for rejection. The ability to assess and predict graft metabolic function could be beneficial in determining appropriate individual immunosuppressant dosing in the posttransplant period. Because of the continuing shortage of suitable cadaveric donor livers for the ever-growing number of pa-

Abbreviations: ACR, acute cellular rejection; Alk Phos, alkaline phosphatase; ALT, alanine aminotransferase; aPTT, activated partial thromboplastin time; AST, aspartate aminotransferase; ESLD, end-stage liver disease; ICG, indocyanine green; IR, ischemia reperfusion; OLT, orthotopic liver transplantation; PDR, plasma disappearance rate; POD, postoperative day; PT, prothrombin time. Supported by the Nihon Khoden Corp. and the Research Programs Council of the Cleveland Clinic. Address reprint requests to Brian M. Parker, M.D., Associate Professor of Anesthesiology, Cleveland Clinic Lerner College of Medicine of Case Western Reserve University, Head, Section of Anesthesia for General Surgery & Liver Transplantation, Department of General Anesthesiology/ E31, Cleveland Clinic, 9500 Euclid Avenue, Cleveland, Ohio 44195. Telephone: 216-444-4136; FAX: 216-444-9247; E-mail: [email protected] DOI 10.1002/lt.21308 Published online in Wiley InterScience (www.interscience.wiley.com).

© 2007 American Association for the Study of Liver Diseases.

IMMUNOSUPPRESSANT DOSING AFTER TRANSPLANTATION 47

tients requiring orthotopic liver transplantation (OLT), the use of extended criteria donors (as assessed by age, degree of IR injury, and surgical reduction such as split livers and living donor liver transplantation) is increasing.4 As a result, several investigators have examined approaches to individualized dosing of immunosuppressive agents by optimizing their therapeutic effect while minimizing their toxic effects, using clearance of hepatically metabolized drugs in the postoperative period.5 Evaluation of genetic polymorphism as a potential guide to determine the dosing/serum drug concentration relationship is a promising modality; however, it has not yet been applied clinically.5 Measuring the plasma disappearance rate (PDR) of the dye indocyanine green (ICG) is another modality that can also be used to determine postoperative liver allograft function and potentially its metabolic activity. ICG is unique in that this water-soluble molecule, once injected intravenously, is extracted from the systemic circulation by the liver exclusively and is subsequently excreted into the bile. When administered in a bolus dose of 0.5 mg/kg or less, ICG exhibits first-order kinetics.6,7 Thus, the fractional plasma elimination rate and the half-life of ICG are both constant. In this study, noninvasive ICG-PDR was measured in order to (1) assess the relationship between measured ICG-PDR and the ability of patients to achieve therapeutic immunosuppressive blood levels, (2) assess the relationship between ICG-PDR and obtained postoperative liver biopsy results, and (3) study the correlation between ICG-PDR and liver biochemical markers. We hypothesized that noninvasive measurement of ICGPDR would allow for early determination of higher dosing tacrolimus requirements to achieve therapeutic serum levels in patients after OLT.

PATIENTS AND METHODS After institutional review board approval and informed consent were obtained, 20 adult patients (age ⬎ 18 years) with end-stage liver disease scheduled for whole organ cadaveric OLT were enrolled in this prospective, nonrandomized clinical study. Patients with significant cardiovascular disease, preoperative hemodynamic instability, or sepsis were not enrolled. ICG clearance was assessed with the DDG-2001 analyzer (Patient Monitoring Systems Division, Nihon Khoden Corp., Tokyo, Japan). The use and validation of this technology in evaluating hepatic function in liver transplant recipients have been described elsewhere.6,8-10 The DDG-2001 analyzer constructs a dye densitogram (graphical representation) of serum ICGPDR with a noninvasive optical pulse-spectrometry nasal probe. This graph of ICG-PDR is actually a decay curve, with the slope designated as k. Thus, k represents the rate of disappearance of ICG from the blood as the liver extracts it from the circulation. The smaller the k value is, the lower the rate is of extraction of ICG from the blood. The analyzer determines the concentration of ICG (mg/L) by comparing the infrared absorption spectra of ICG at both 810 and 940 nm; this is similar to the

methods used in pulse oximetry. In this study, a single 0.5 mg/kg intravenous bolus of ICG was administered to patients at two different times following reperfusion via indwelling central venous cannulae to assess allograft function. All ICG injections were performed over a period of no more than 3 seconds. All patients had a baseline allograft liver function determined with the DDG-2001 analyzer 2 hours after reperfusion of the allograft prior to the end of surgery. In addition, allograft liver function determined via ICGPDR measurement was also assessed on postoperative day (POD) 1 at 18 hours post reperfusion. For each liver function trial using the DDG-2001, ICG decay curves and k values (slope of the decay curve) were determined. Patients’ baseline (2 hours post reperfusion) allograft ICG-PDR k values were compared to POD 1 (18 hours post reperfusion) ICG-PDR k values. In addition, each patient’s baseline ICG-PDR k value was compared to the standard “normal” ICG-PDR k value (⬎0.19) with Spearman correlations to determine if “normal” allograft function was present.11 Both intraoperative and postoperative hemodynamic data [systemic arterial blood pressure (mm Hg), central venous pressure (mm Hg), pulmonary artery pressure (mm Hg), and cardiac output (L/min)] at the time of each ICG-PDR determination were analyzed to ensure that ICG measurements were conducted during periods of hemodynamic stability. Standard biochemical tests of liver function were also obtained at each ICG-PDR determination, including aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase, total bilirubin, albumin, prothrombin time (PT), activated partial thromboplastin time (aPTT), and platelet count. Abdominal Doppler ultrasound examinations were performed on POD 1 to examine patency of both the portal vein and hepatic artery prior to ICG injection. Each patient’s postoperative immunosuppressive protocol was not dictated by this study but was individualized and determined by the staff surgeon postoperatively. Tacrolimus dosing of 0.1 mg/kg was begun on POD 1 in two divided doses administered 12 hours apart. Immunosuppressant dosing was adjusted to attain a stable serum trough level of ⬎12 ng/mL prior to patient discharge from the hospital. Data regarding each patient’s immunosuppressive regimen (daily dose and daily serum concentration of tacrolimus) during the hospital stay were collected. In addition, the result of each patient’s postoperative liver biopsy (obtained prior to POD 7) was graded as no rejection, mild acute cellular rejection (ACR), moderate ACR, or severe ACR. Lastly, standard organ donor data were collected and analyzed, including age, gender, cytomegalovirus status, use of vasopressors at the donor institution, cause of brain death, and organ cold and warm ischemic times. Primary statistical testing compared allograft metabolic function at the two ICG-PDR measurement times: intraoperatively at the end of surgery and on POD 1. Descriptive statistics for ICG-PDR k values and standard biochemical tests of liver function were also cal-

LIVER TRANSPLANTATION.DOI 10.1002/lt. Published on behalf of the American Association for the Study of Liver Diseases

48 PARKER ET AL.

TABLE 1. Patient Demographic Data Age (years)* Weight (kg)* Height (cm)* Male [n (%)] Cause of ESLD [n (%)] Autoimmune hepatitis Cryptogenic cirrhosis Hepatitis C Hepatitis C and Laennec’s cirrhosis Laennec’s cirrhosis Methotrexate-induced cirrhosis Nonalcoholic steatohepatitis Primary biliary cirrhosis

51.6 (7.4) 85.6 (7.7) 172.3 (7.6) 14 (70) 1 (5) 3 (15) 5 (25) 3 (15) 3 (15) 2 (10) 2 (10) 1 (5)

Abbreviation: ESLD, end-stage liver disease. * Data are presented as mean (standard deviation).

culated for each of these time points. Either the paired t test or Wilcoxon signed-rank test was used to compare differences in these factors, including ICG-PDR k values from the end of surgery to POD 1. Spearman correlations were calculated to assess the relationship between measured ICG-PDR k values and standard liver function tests. Fisher’s exact test was used to evaluate recipient ICG-PDR k values and cadaveric liver donor factors. Data are presented as either mean ⫾ standard deviation or median with percentiles (25th, 75th). A multinomial logit model was used to assess whether the ICG-PDR k values 2 or 18 hours post reperfusion were associated with postoperative biopsy results (no, mild, or moderate/severe rejection). In addition, mixed model analysis of variance was performed to assess trends in tacrolimus blood level and in tacrolimus dose by POD. Day of surgery and POD 1 ICG-PDR k values, linear and quadratic effects of the number of days post transplant, and postoperative liver biopsy results were potential predictors considered in these analyses, as were all two-way interactions. A P value of less than 0.05 was considered statistically significant.

RESULTS During the patient acquisition phase of this study, a total of 25 patients were enrolled. However, 5 of these patients were subsequently removed from analysis. None of these removed patients underwent OLT after study enrollment because the donor organ was felt to be suboptimal at that time by the surgical team. Thus, a total of 20 study patients were analyzed. The intraoperative management, including anesthetic choice and surgical technique, was not dictated by the study protocol, although the surgical approach (including the use of venovenous bypass), hemodynamic goals, and pharmacologic agents used were similar in all cases. Demographic data for all patients are presented in Table 1. Select hemodynamic data collected immediately prior to each of the two ICG administrations are presented in Table 2. The median ICG-PDR k value for the study group at 2 hours post reperfusion was 0.20

(0.16, 0.27), whereas at 18 hours post reperfusion, it was 0.22 (0.18, 0.35). The median change in the k value between the two ICG-PDR measurements was 0.05 (⫺0.02, 0.07) with P ⫽ 0.02. In addition, 7 patients were observed to have markedly elevated k values. These 7 patients had median k values of 0.28 (0.25, 0.33) at 2 hours post reperfusion versus 0.16 (0.15, 0.21) in the remaining 13 patients (P ⫽ 0.03) and median k values of 0.40 (0.32, 0.42) at 18 hours post reperfusion versus 0.20 (0.15, 0.22) in the other 13 patients (P ⫽ 0.002). Individual line plots of patient ICG-PDR k values are shown in Fig. 1. Of the 20 study patients enrolled, 16 were administered tacrolimus in combination with a steroid postoperatively. Four patients were given sirolimus postoperatively because of renal dysfunction noted at the time of OLT. One patient was started on tacrolimus but converted to sirolimus on POD 10 because of postoperative renal dysfunction. Only tacrolimus blood level data were acquired through POD 15. Of the 16 patients that received tacrolimus, comparable dosing on a per kilogram body weight basis was observed (0.964 ⫾ 0.339 mg/kg), regardless of the ICG-PDR k values measured after reperfusion. In the 7 patients noted to have markedly elevated ICG-PDR k values, mean tacrolimus dosing was 1.127 ⫾ 0.336 mg/kg. Although slightly higher dosing was observed in this subgroup of patients, it did not achieve statistical significance. Mixed model analysis of variance revealed an interaction between the POD 1 (18 hours post reperfusion) ICG-PDR k value and the linear increase in the tacrolimus blood level (Fig. 2), such that the greater the k value was, the more gradual the observed rise was in tacrolimus over time [that is, the longer it took to achieve a therapeutic blood level (⬎12 ng/mL), P ⫽ 0.003]. Lastly, immunosuppressant dosing for both tacrolimus and sirolimus were observed to increase until POD 12 and then began decreasing (statistically significant linear and quadratic effects of time, P ⬍ 0.0001). This observation was independent of ICG-PDR k values or postoperative biopsy results. Postoperative liver biopsy results for all study patients were as follows: no rejection (n ⫽ 7), mild ACR (n ⫽ 7), and moderate to severe ACR (n ⫽ 6). No significant association between ICG-PDR k values and postoperative liver biopsy results was observed. Weak correlations were observed between measured ICG-PDR k values 18 hours post reperfusion and serum alkaline phosphatase at both 2 hours and 18 hours post reperfusion (Table 3). Correlations between ICGPDR k values and other standard biochemical tests of liver function (PT, aPTT, and platelet count) were not observed (Table 3). Lastly, no associations were found between recipient ICG-PDR k values, standard biochemical tests of liver function (PT, aPTT, and platelet count), or postoperative liver biopsy results with any cadaveric liver donor factors. Transabdominal Doppler ultrasound examinations of each recipient’s portal vein and hepatic artery were conducted on POD 1 prior to the second ICG-PDR measurement. All examinations revealed patency with ade-



TABLE 2. Select Patient Hemodynamic Data Obtained at Both 2 and 18 Hours Post Reperfusion

Heart rate (beats per minute) Systolic blood pressure (mm Hg) Central venous pressure (mm Hg) Cardiac output (L/minute)

2 Hours Post Reperfusion

18 Hours Post Reperfusion

85.0 (70.5, 91.5) 111.0 (99.5, 125.5) 9.0 (7.5, 14.0) 10.1 (8.0, 13.0)

83.0 (68.5, 98.0) 134.0 (123.0, 149.5) 10.5 (9.0, 15.0) 9.1 (8.0, 11.4)

NOTE: Data are presented as median with percentiles (25th, 75th).

Figure 1. Individual line plots of ICG-PDR k values at both 2 and 18 hours post reperfusion for all 20 study patients. Abbreviation: ICG-PDR, indocyanine green plasma disappearance rate.

quate blood flow in both vessels. Finally, no patients died and no grafts were lost during the study period.

Figure 2. Rate of increase of postoperative tacrolimus blood levels (ng/mL) versus ICG-PDR k values 18 hours post reperfusion for 16 patients following OLT. Points represent estimated slope parameter from regressing tacrolimus serum levels on the ICG-PDR k values at 18 hours for each patient. The solid line represents the linear regression fit through these points, that is, the average linear trend over all patients. Dashed lines represent 68% confidence levels, that is, one standard deviation from the estimate line. Abbreviations: ICGPDR, indocyanine green plasma disappearance rate; OLT, orthotopic liver transplant.

DISCUSSION Immediate postoperative OLT immunosuppressive drug therapy is guided by protocols based on pharmacokinetic data. However, individual variation in graft function in the postoperative period can potentially make these protocols not applicable to all patients, especially with dynamic changes in liver function and when immunosuppressive agents such as tacrolimus depend heavily on hepatic elimination. The narrow therapeutic margin of tacrolimus puts transplant patients at risk of overimmunosuppression with associated toxicity or inadequate immunosuppression resulting in episodes of rejection.12 Understanding the pharmacokinetics of tacrolimus has allowed the development of dosing protocols based on blood level monitoring. However, even with strict adherence to the protocols, the blood concentration of tacrolimus poorly correlates with the dosage because of interpatient and intrapatient variability in absorption, distribution, and elimination; therefore, it is mandatory to follow the drug concentration. For the same reasons, the rate of tacrolimus blood concentration rise is impossible to predict as well. Tacrolimus is extensively metabolized in the liver by the cytochrome P450 3A enzymes12; hence, liver

(allograft) metabolic function has a significant impact on the drug’s pharmacokinetics.3 The very early diagnosis of allograft dysfunction following OLT remains a significant area of interest. The differential diagnosis of graft dysfunction in the immediate postoperative period includes but is not limited to IR injury, acute rejection, and occlusion of the hepatic vasculature. Postoperative acute rejection may be related to either not achieving immunosuppression or immunosuppression occurring at too slow a rate. The primary aim of this study was to determine if noninvasive ICG-PDR measurements predicted the rate of rise of immunosuppressant agent serum levels in patients following OLT. To date, no prospective clinical study measuring intraoperative noninvasive ICG elimination during or immediately after OLT has been presented correlating graft metabolic function and ability to achieve adequate immunosuppressant drug levels with standard dosing regimens. However, the use of arterial fiber-optic thermistor catheters in post-OLT patients10,13 and several case reports using noninvasive ICG elimination measurements during OLT have been described.14,15


50 PARKER ET AL.

TABLE 3. Correlations Between Measured ICG-PDR k Values and Patient Serum Tests at Both 2 and 18 Hours Post Reperfusion ICG-PDR k Value at 2 Hours Post Reperfusion Serum tests at 2 hours post reperfusion AST ALT Alk Phos Total bilirubin Albumin Prothrombin time aPTT Platelet count Serum tests at 18 hours post reperfusion AST ALT Alk Phos Total bilirubin Albumin PT aPTT Platelet count

ICG-PDR k Value at 18 Hours Post Reperfusion

⫺0.22 0.01 ⫺0.25 ⫺0.14 0.44 ⫺0.27 ⫺0.09 0.08

(⫺0.7, (⫺0.5, (⫺0.7, (⫺0.6, (⫺0.0, (⫺0.7, (⫺0.6, (⫺0.4,

0.3) 0.5) 0.2) 0.3) 0.9) 0.2) 0.4) 0.6)

⫺0.41 (⫺0.9, 0.0) ⫺0.30 (⫺0.8, 0.2) ⫺0.11 (⫺0.6, 0.4)* ⫺0.52 (⫺0.9, ⫺0.1) 0.28 (⫺0.2, 0.8) ⫺0.26 (⫺0.7, 0.2) ⫺0.20 (⫺0.7, 0.3) ⫺0.07 (⫺0.6, 0.4)

⫺0.40 0.11 ⫺0.21 ⫺0.20 0.21 ⫺0.09 ⫺0.22 0.28

(⫺0.9, (⫺0.4, (⫺0.7, (⫺0.7, (⫺0.3, (⫺0.6, (⫺0.7, (⫺0.2,

0.1) 0.6) 0.3) 0.3) 0.7) 0.4) 0.3) 0.8)

⫺0.40 (⫺0.9, 0.1) ⫺0.19 (⫺0.7, 0.3) ⫺0.13 (⫺0.6, 0.4)† ⫺0.58 (⫺1.0, ⫺0.2) ⫺0.11 (⫺0.6, 0.4) ⫺0.26 (⫺0.7, 0.2) ⫺0.15 (⫺0.6, 0.3) 0.21 (⫺0.3, 0.7)

NOTE: Data are presented with Spearman correlations: rho (25th, 75th). Abbreviations: Alk Phos, alkaline phosphatase; ALT, alanine aminotransferase; aPTT, activated partial thromboplastin time; AST, aspartate aminotransferase; ICG-PDR, indocyanine green plasma disappearance rate; PT, prothrombin time. * P ⫽ 0.018. † P ⫽ 0.007.

Because ICG elimination requires both adequate hepatic blood flow and allograft function for uptake, we hypothesize that ICG may be an indicator for assessing instantaneous allograft function and hence be a surrogate predictor for hepatic metabolism of immunosuppressant agents. Liver aminotransferase measurements, although widely accepted as indicators of hepatic function, also represent in part the degree of damage to hepatocytes (for example, cold or warm ischemic time and IR injury) and thus do not necessarily provide immediate or accurate information regarding remaining allograft function. Also, those tests that assess the elimination (that is, uptake) of markers such as ICG may be more sensitive and allow for earlier detection of allograft dysfunction than those tests relying on enzymatic release after hepatocyte damage or hepatic synthetic activity. The estimation of hepatic synthetic activity by the measurement of PT, aPTT, or fibrinogen levels can also be affected by transfusion of blood products in the perioperative period, and this makes it difficult to evaluate the contribution of the newly transplanted liver. Importantly, significant metabolism of ICG does not occur. Thus, only hepatic blood flow, dye extraction, and biliary transport are responsible in determining ICG elimination. However, using ICG to evaluate liver function does not allow one to distinguish between hepatic blood flow and the uptake and excretory phases of ICG elimination.16 Therefore, the performance of a transabdominal hepatic vascular Doppler ultrasound examination is necessary in the immediate postoperative period to eliminate significant changes in

hepatic blood flow via either the portal vein or hepatic artery as a cause for alterations in ICG elimination. In this study, all patients were observed to have adequate hepatic inflow at the conclusion of surgery as assessed by the surgical team, and on POD 1, hepatic vessel patency and flow within the vessels were confirmed. However, hepatic blood flow may still have played an important role in the observed ICG-PDR measurements obtained. Seven patients were noted to have significantly increased ICG-PDR k values both intraoperatively and postoperatively (2 and 18 hours post reperfusion, respectively) in comparison with the rest of the study group, such that these values could be considered supranormal. Despite this interesting finding, no significant correlation was found between these supranormal k values and any specific donor or recipient factors. It is likely that the supranormal ICG-PDR was due to postischemic hyperemia, which has previously been described.10,17 In the setting of OLT, immediately following reperfusion, an increase in cardiac output of up to 20% can be seen contributing to the observed graft hyperemia. Interestingly, these supranormal k values persisted into POD 1 when the second ICG-PDR measurement was obtained. In this study, patient CO values did not change significantly between the two ICGPDR measurements taken at 2 and 18 hours post reperfusion, respectively. It is possible that the postischemic hyperemia continued into the postoperative period; however, hemodynamic data obtained during ICG-PDR measurements do not support this notion.18



As described by Plevris et al.,19 inflammatory changes following reperfusion of the allograft may contribute to a decreased ability of hepatocytes to excrete ICG due to a decrement in oxygen delivery as a result of microcirculatory changes. In our patients, no correlation was observed between ICG-PDR k values and postoperative liver biopsy results, and no grafts were lost. Thus, it is possible that hyperemia and/or enhanced oxygen delivery to the allografts may have contributed to the elevated ICG-PDR k values seen at both 2 and 18 hours post reperfusion in this subgroup of study patients. Although the exact cause for these observed supranormal ICG-PDR k values is unknown at this time, it may also be related to increased metabolic activity of the allograft. As observed in this study, patients with grafts having supranormal ICG elimination k values required a longer time to achieve therapeutic serum levels of tacrolimus. This fact may have significant importance because there is strong evidence that there is an inverse relationship between increased tacrolimus serum trough levels and a decreased risk of acute rejection postoperatively.12 In this study, postoperative liver biopsy results were evaluated for all patients; however, ICG-PDR k values above normal did not predict the presence of acute rejection. This lack of correlation is likely related to the relatively small study population and even smaller subgroup of patients exhibiting supranormal k values. Of note, the mean dosing tacrolimus value per kilogram of body weight in the supranormal k value subgroup was slightly higher (but not statistically significant) in comparison with the rest of the study patients. Thus, the rate of rise of achieving a therapeutic serum drug level was more gradual in these 7 patients, despite somewhat more tacrolimus being administered to them. There are several limitations of this study that should be kept in mind when the results are interpreted. The authors acknowledge the small sample size of the study group and that only 16 patients were analyzed with regard to tacrolimus levels after OLT. In addition, in studies with small sample sizes such as this, it can be difficult to detect significant effects, and wide confidence intervals can be observed (see Fig. 2). Also, it is important to recognize that activity of the cytochrome P450 3A enzyme system, which metabolizes tacrolimus, can be affected by multiple donor and recipient factors, including donor organ quality, quality of preservation, IR injury, the recipient’s proinflammatory response (that is, induction or suppression of the cytochrome P450 3A system by cytokines), and concomitant medication administration. Although some factors could be accounted for in our analysis, some could not, while others were difficult to quantify (for example, donor organ quality and degree of IR injury). As previously mentioned, hepatic blood flow was not precisely measured during the study; thus, there is a possibility that variations of what is considered “adequate” blood flow could affect the metabolism of both ICG and tacrolimus. In addition, it is unknown whether the metabolism of each drug was affected to the same

degree. Lastly, all study patients did well in the immediate postoperative period with no graft loss due to either primary nonfunction or acute rejection. As a result, the spectrum of possible patient outcomes following OLT is not fully represented in this study. In conclusion, this study demonstrates that ICG-PDR measurement is a modality with the potential to assist clinicians in achieving adequate serum levels of tacrolimus after OLT. The noninvasive optical pulse-spectrometry method provides a relatively quick and easy “bedside” method to quantify ICG-PDR. By confirming that the allograft blood supply is patent with adequate flow via abdominal Doppler ultrasound, the ICG-PDR k value may aid in identifying those patients requiring higher initial tacrolimus dosing than what is currently standard practice. Because this study detected no association between the slower rise of tacrolimus serum levels and evidence of acute rejection on biopsy, further studies involving a greater number of patients are needed to evaluate whether measurement of ICG-PDR may serve as a guide for the postoperative immunosuppression dosing regimen. Importantly, larger studies12 have clearly demonstrated the relationship between increasing serum trough levels of tacrolimus and decreasing risk of acute rejection. Thus, the ability to identify patients who require higher, more aggressive tacrolimus dosing by noninvasive ICG-PDR on POD 1 may potentially decrease the incidence of acute rejection episodes related to the inability to achieve rapid tacrolimus therapeutic serum levels.

ACKNOWLEDGMENT The authors gratefully acknowledge Mr. Dan Paloski for his editorial assistance.

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