Cost effectiveness of pharmacogenetic testing for UGT1A1 before irinotecan administration for metastatic colorectal cancer

Abstract

Purpose

To examine the cost effectiveness of using a pharmacogenetic test for UGT1A1*28 variant homozygosity before administering irinotecan in metastatic colorectal cancer patients. Patients and Methods: Decision-analytic model from Medicare payer perspective followed hypothetical patients treated with FOLFIRI (5-fluorouracil/leucovorin with irinotecan). Under usual care, patients received a full dose of irinotecan. With genetic testing, irinotecan dosage was reduced 25% in homozygotes with the UGT1A1*28 variant allele. Test performance, chemotherapy toxicity, and quality-of-life weights were from clinical literature and product labels. Costs were from 2007 Medicare fee schedules. Chemotherapy efficacy after dose reduction, adverse event risk, and other parameters were varied in one-way and probabilistic sensitivity analyses. We calculated the value of investing in further studies of chemotherapy efficacy following homozygote dose reductions.

Results

Pre-treatment genetic testing costs less ($272 savings/patient tested) and yields slightly improved quality-adjusted life expectancy (0.1 quality-adjusted day/patient tested, approximately 2 quality-adjusted hours). Results depend on treatment efficacy but not adverse event risk assumptions. Testing would avoid 84 cases of severe neutropenia including 4.4 deaths. At a threshold of $100,000 per quality-adjusted life year, therapeutic efficacy of irinotecan in homozygotes after dose reduction must be ≥98.4% of full-dose efficacy for genetic testing to remain preferred. Future studies to determine whether this efficacy level can be achieved have an economic value of $22 million.

Conclusion

Pharmacogenetic testing for UGT1A1*28 variant homozygosity may be cost effective, but only if irinotecan dose reduction in homozygotes does not reduce efficacy. Future studies to evaluate reduced-dose efficacy in homozygotes should be considered.

Introduction

In 2008, colorectal cancer was diagnosed in over 150,000 men and women in the United States.¹ Despite advances in screening and early detection, nearly 20% of patients present with metastatic disease.¹ Many others will develop recurrent and/or unresectable colorectal cancer after completing adjuvant therapy and will be eligible for palliative chemotherapy to control their disease. While oxaliplatin-based combination chemotherapy regimens (e.g., FOLFOX+/− bevacizumab) are currently favored for first-line treatment, regimens containing irinotectan (Camptosar©) are accepted and widely used in the first-, second-, and third-line metastatic setting.^{2, 3}

Neutropenia is a serious side effect of irinotecan treatment, particularly in patients with decreased uridine diphosphate (UDP)-glucunorosyltransferase activity.⁴ Administered as a prodrug, irinotecan undergoes enzymatic conversion by carboxyesterase-2 to yield the clinically active metabolite, SN-38. This active form interferes with tumor cell division by inhibiting the nuclear enzyme topoisomerase I.⁵ SN-38 is eliminated from the body through the biliary system, after a process of glucuronidation (conjugation to glucuronic acid) via the uridine diphosphate glucuronosyltransferase (UGT)1A1 enzyme. Several retrospective studies have demonstrated that individuals who are homozygous for UGT1A1*28 are at increased risk of irinotecan-related neutropenia and diarrhea, particularly when irinotecan is administered at higher doses.^{4, 6–9} This likely is due to markedly reduced mRNA transcription leading to decreased overall enzyme production, slower SN-38 glucuronidation, and a greater SN-38 plasma concentration over time, resulting in prolonged exposure to the unconjugated form of SN-38.¹⁰ As many as 11% of North Americans may be homozygous for this allele, although the prevalence is known to be lower among Asian-Americans and African-Americans compared to Caucasians.⁵

In 2005 the FDA approved a UGT1A1 genotype test “as an aid in the identification of patients with greater risk for decreased UDP-glucunorosyltransferase activity” ¹¹ and therefore with an increased risk of irinotecan-induced neutropenia. The manufacturer’s label for irinotecan now recommends that physicians “consider” a dose-reduction of “at least one level” (equivalent to approximately a 17–34% dose reduction) when administering the drug to patients who are known to be homozygous for UGT1A1*28, but notes that “the precise dose reduction in this patient population is not known.”¹²

Data regarding the effects of irinotecan dose reduction on the incidence of side effects or overall survival in colorectal cancer patients are limited. A recent meta-analysis indicated that lower doses (<150 mg/m 2) versus medium (150 – 250 mg/m 2) and high (>250 mg/m 2) doses were associated with a lower probability of severe neutropenia in homozygotes.⁹ In addition, little is known about the costs associated with implementing UGT1A1 testing. Indeed, few cost-effectiveness studies have been conducted of pharmacogenetic tests for inherited mutations of any kind.¹³ The purpose of this study was to perform an economic evaluation of the use of pharmacogenetic testing to guide dosing of irinotecan versus irinotecan administration according to usual care in metastatic colorectal cancer patients.

Methods

Model Overview

We created a decision analytic model from the Medicare payer perspective using TreeAge Pro 2008 (Williamstown, MA)¹⁴ to evaluate the cost effectiveness of testing for the UGT1A1*28 genotype prior to irinotecan administration in metastatic colorectal cancer patients compared to usual care (no testing). (Figure 1.) Hypothetical patients in the model were treated with FOLFIRI (5FU/LV with irinotecan), with a standard intermediate dose (175mg/m²) calculated for a patient with body surface area of 1.85m² (e.g., 5′10″, 70kg person).^{15, 16} We chose to study the FOLFIRI regimen for the model because it is the irinotecan-containing regimen for which the most data on UGT1A1*28 testing and outcomes are currently available. In the usual care strategy, all patients received a standard intermediate dose of irinotecan. In the genetic testing strategy, patients underwent UGT1A1*28 testing prior to FOLFIRI administration. Those who were found to be homozygous for the UGT1A1*28 variant allele received a 25% dose reduction of irinotecan therapy to 131.25 mg/m², based on the FDA’s label suggesting a dose reduction of at least one level, whereas all others received the full dose.

Figure 1.

Decision analytic model

We began by assuming that the FOLFIRI regimen had 100% efficacy in all patients and had the same efficacy at a reduced dose among homozygotes as a full dose among heterozygotes or wild types, because there is slower glucuronidation and a greater plasma concentration of SN-38 in homozygotes.⁶ This assumption was then varied along with other model inputs in one-way and probabilistic sensitivity analyses. A recent study indicated that response rates were not different between homozygotes, heterozygotes, or wild types, even though homozygotes experienced more frequent dose reductions due to adverse events.¹⁷

We report incremental costs and quality-adjusted life expectancy when comparing the two strategies and the economic value of additional investment to reduce uncertainties in the modeling assumptions. We also report the estimated number of severe neutropenia events and deaths avoided by testing.

Adverse Events and Life Expectancy

Severe adverse events associated with irinotecan include grade 3/4 (severe) neutropenia.^{18, 19} The risks of severe neutropenia were derived from randomized trials of irinotecan administration in genotyped populations (Table 1).^{8, 20} Among homozygotes, the probability of severe neutropenia declined from 14% to 3.7% after genetic testing and dose reduction (versus 3.7% in the rest of the population), based on a study indicating that such dose reductions decreased the risk of a severe adverse event to a risk that was statistically equivalent to that of heterozygotes and wild types (p=0.2).⁸ We reduced this benefit in sensitivity analyses. A fraction (23%) of patients experiencing severe neutropenia were assumed to be admitted to the hospital for management of febrile neutropenia,²¹ and the remainder were assumed to be treated on an outpatient basis. Because of the very small number of deaths due to adverse events reported in each trial,^{15, 22–26} we assumed the probability of death during a hospitalization for severe or febrile neutropenia was 0.1% (Table 1). Life expectancy following FOLFIRI treatment was assumed to be 24 months.²⁷

Table 1.

Variable assumptions

Variable name	Initial Value	Range	Source
Adverse Events
Probability of severe neutropenia, homozygotes with full dose	0.14	0.04–0.70	Toffoli8, Rouits18
Probability of severe neutropenia, wild types and heterozygotes and homozygotes with reduced dose	0.037	0.03–0.23	Weighted average based on population prevalence; Toffoli8, Rouits18
Probability of hospitalization with severe neutropenia	0.23	0.23–1	Elting21
Probability of death from neutropenia	0.001	0–0.03	Trials15, 16, 22, 24 and expert opinion
Genetic Test Characteristics
UGT1A1 assay sensitivity	0.9999	0.90–1	Food and Drug Administration
UGT1A1 assay specificity	0.9999	0.92–1	Food and Drug Administration
Prevalence of *28 allele, homozygous	0.11	0.01–0.20	Lin30
Prevalence of *28 allele, heterozygous	0.43	0.40–0.46	Lin30
Quality of Life
Metastatic colorectal cancer	0.83	0.8–1.0	Brown ML31
Severe neutropenia	0.42	0.25–0.55	Brown RE32
Efficacy
Efficacy of reduced dose (75%) of FOLFIRI in homozygotes	100%	75–100%	Assumption
Life expectancy	24 months	6–24 months	Fuchs27
Costs
Cost of UGT1A1 test	$102.83	$50–750 [$75 for societal perspective]	Medicare Clinical Laboratory Fee Schedule17
Cost of physician office visit	$69		Medicare CPT 9921436
Hospitalization for severe neutropenia	$9500		2007 DRG weight*$7500 base rate35 (DRG 574)
Chemotherapy, full dose	$12950		Medicare Drug Acquisition Price (2007)37; multiplied by 75% for reduced dose

Test characteristics and allele prevalence

Genetic testing was assumed to be conducted using the Invader molecular assay developed by Third Wave Technologies (Madison, WI), an FDA-approved test for UGT1A1*28 genotyping.²⁸ Sensitivity and specificity of the assay were reported on the FDA website as 100%.²⁹ However, we assumed sensitivity and specificity of 99.99% in order to allow for a small chance of a laboratory error that would affect test results. In the North American population, the prevalence of homozygotes was estimated at 11%.³⁰

Quality of Life and Costs

We used previously published quality-of-life weights (utility values) associated with metastatic colorectal cancer and severe neutropenia (Table 1). A baseline weight of 0.83 was used for the population of metastatic colorectal cancer patients.³¹ Severe neutropenic events were assumed to last one week, leading to an additional decrement in quality of life during that week. Febrile neutropenia alone has been reported to be associated with a quality-of-life value of 0.42 (equivalent to a utility decrement of 0.58 from perfect health).³² The quality of life of patients experiencing both metastatic colorectal cancer and a severe neutropenic event (febrile neutropenia) simultaneously was estimated at 0.42 during one week, using a lowest-value approach.³³

We used the Medicare payer perspective to evaluate costs. The model included costs of UGT1A1*28 testing,³⁴ costs of hospitalization for febrile neutropenia,³⁵ cost of a physician office visit,³⁶ and costs of FOLFIRI chemotherapy for a full dose in wild types and heterozygotes and for a reduced dose in homozygotes.³⁷ The model did not include costs of subsequent chemotherapy, because the wide variety of possible treatments available impaired our ability to calculate this cost with an acceptable degree of accuracy. We wanted to estimate the maximum amount of future medical costs that might be incurred as a result of patients living longer due to the use of the test while still having the test be cost effective. We therefore estimated the threshold value at which the additional lifetime medical costs due to improved survival as a result of genetic testing would be offset by the benefits of testing. All costs were reported in 2007 U.S. dollars. Quality-adjusted life expectancy and costs were discounted at an annual rate of 3% per year, in accordance with the recommendation of the Panel on Cost-effectiveness in Health and Medicine.³⁸

Sensitivity analyses

We performed one-way sensitivity analyses on the efficacy of FOLFIRI dose reduction, incidence of and mortality due to severe neutropenic events, genetic test performance, cost, and quality of life model inputs for a specified range of values (Table 1). Of note, we varied the efficacy of a dose reduction from 75–100% of full dose efficacy; in the model, a reduction in efficacy resulted in a proportional reduction in life expectancy after FOLFIRI treatment (e.g., a 75% reduction in efficacy resulted in a reduction in life expectancy from 24 months to 18 months). We also considered different methods for combining quality-of-life weights for severe neutropenia and metastatic colorectal cancer.³³

We evaluated the impact of substituting a societal perspective for the Medicare payer perspective by adding $900 to reflect the value of patient time associated with hospitalization³⁹ and reducing the cost of the UGT1A1*28 test from $103 (the Medicare reimbursement rate) to $75, which represented the estimated cost to conduct the test in a hospital laboratory.

Value of Information

We conducted a probabilistic sensitivity analysis by running 1000 Monte Carlo simulations that varied all non-cost variables simultaneously with a willingness-to-pay (WTP) threshold of $100,000/quality-adjusted life year (QALY).⁴⁰ We applied triangular distributions⁴¹ for all parameters, where the peak was equal to the initial value and the lower and upper bounds were equal to the lowest or highest values found in the literature. The exceptions to triangular distributions were those for which data were very limited: chemotherapy efficacy, quality-of-life weights, and probability of hospitalization for neutropenia. We selected uniform distributions for these parameters, because they reflect the highest possible degree of uncertainty and are the least “informative” as they represent the distribution of possible values as a straight line within a range.

Using the results of the probabilistic sensitivity analysis with a WTP threshold of $100,000/QALY, we estimated the value of obtaining better quality data to reduce uncertainties in our model, and therefore the value that represents the maximum amount that should be spent to conduct a study of dose reductions in homozygotes. First we calculated the expected value of perfect information (EVPI). EVPI can be interpreted as the value of completely removing the possibility of making an incorrect decision based on the current analysis, due to its inherent uncertainty.⁴² We then calculated the expected value of partial perfect information (partial EVPI) for resolving just the uncertainty about the efficacy of a reduced dose of irinotecan in homozygotes. The partial EVPI represents the maximum value society should be willing to pay to eliminate the uncertainty about the efficacy assumption alone. We assumed that the test would continue to be useful for 5 years before being replaced by new technologies, and that the size of the population to which the test can be applied is 29, 260 metastatic colorectal cancer patients annually,²¹ 63% of whom we estimate are age 65 and older based on SEER data from 1973–2002.⁴³ Finally we compared the partial EVPI to how much a clinical study might cost to improve decision making concerning dose reductions.

Results

Adverse events and Cost-effectiveness

For every 10,000 patients tested, 1,100 (11%) would receive a 25% dose reduction as a result of being identified as UGT1A1*28 homozygous. On average, this treatment strategy would avoid 84.5 cases of severe neutropenia (including 4.5 deaths due to neutropenia) and would save $2.7 million in treatment costs. Taking into account the cost of testing and assuming no reduction in treatment efficacy, the average cost savings and quality-adjusted life expectancy savings per patient tested was $272.34 and 0.073 quality-adjusted days (Table 2), not even 2 quality-adjusted hours. If all 29,000 metastatic colorectal cancer patients diagnosed each year in the U.S. were treated with chemotherapy that included irinotecan, the estimated net annual savings could be as high as $7.96 million and 6 quality-adjusted life years, as a result of avoiding 245 severe neutropenic events (including 13 fatal events).

Table 2.

Cost-effectiveness Results Depending on Efficacy of Dose Reduction in Homozygotes.

Strategy	Cost	Incremental Cost	Effectiveness (QALYs)	Incremental Effectiveness	Incremental Cost- effectiveness ratio (ICER)
Test 100% efficacy	$12,786		1.6349
No test	$13,058	$272	1.6347	−0.0002 QALYs = −0.073 days	Dominated
Test 99% efficacy	$12,786		1.6331
No Test	$13,058	$272	1.6347	0.0016 QALYs = 0.584 days	$170,569
Test 98.4% efficacy	$12,786		1.6319
No test	$13,058	$272	1.6347	0.0027 QALYs = 0.99 days	$100,073*
Test 95% efficacy	$12,786		1.6259
No test	$13,058	$272	1.6437	0.0088 QALYs =3.21 days	$30,965
Test 85% efficacy	$12,786		1.6079
No test	$13,058	$272	1.6347	0.0268 QALYs =9.78 days	$10,165
Test 75% efficacy	$12,786		1.5899
No test	$13,058	$272	1.6347	0.0448QALYs =16.35 days	$6,081

^*Note: When efficacy of a reduced dose in homozygotes falls below 98.4%, the no-testing strategy is below the willingness-to-pay threshold of $100,000/QALY.

The results were sensitive to the efficacy of a reduced dose and somewhat sensitive to the cost of the genetic test but not to adverse event risk. In one-way sensitivity analyses, the estimated efficacy of a reduced dose of irinotecan administered to homozygous carriers following testing must be at least 98.4% of full-dose efficacy for testing to remain the preferred strategy at a willingness-to-pay threshold of $100,000/QALY (Table 2), meaning that efficacy in homozygotes must hold nearly constant following a dose reduction. The cost of the genetic test must be less than $395 (versus the current Medicare cost of $103) for testing to remain the preferred strategy at a WTP threshold of $100,000/QALY; at less than $375, testing both cost less and increased QALYs. Results were not sensitive to the range of values tested for adverse events (incidence of and mortality due to severe neutropenia); for example, if the probability of severe neutropenia after dose reduction in homozygotes was 10.5% (25% reduction) versus 3.7%, the threshold for treatment efficacy of a reduced dose was 98.3%. Results were also not sensitive to life expectancy, genetic test performance (i.e., sensitivity and specificity), and quality-of-life weights (data not shown). Adopting a societal perspective by incorporating patient time costs and a lower test cost did not change the optimal strategy or trends in the sensitivity analyses.

The threshold analysis indicated that in order for genetic testing to remain the dominant strategy, the average cost for the 13 patients who would avoid death due to a severe neutropenic event cannot exceed $618,955 per patient during their remaining life expectancy after completion of the FOLFIRI regimen.

In probabilistic sensitivity analysis, testing dominated usual care in only 9.1% of simulations. Usual care was preferred in 72% of simulations, with an additional 7.5% where usual care resulted in an increase in QALYs but was above the WTP threshold of $100,000/QALY (Figure 2).

Figure 2. Probabilistic Sensitivity Analysis with Willingness-to-Pay (WTP) Threshold of $100,000/QALY.

Note: All non-cost variables were varied according to triangular or uniform distributions (see text). Testing costs less and is more effective in 9.1% of simulations. Usual care was preferred in 72.0% of simulations. X and Y axes are indicated by horizontal and vertical lines, respectively, where they cross zero.

Value of Studies to Reduce Uncertainties

The partial EVPI for the efficacy variable was $22 million over the next 5 years. In other words, a research investment of up to $22 million to evaluate the efficacy of irinotecan dose reduction in homozygotes would be justified based on a WTP threshold of $100,000/QALY if irinotecan remained an accepted and widely-used treatment alternative for metastatic colorectal cancer patients in the United States for the next 5 years. Given that Medicare primarily provides health insurance coverage for patients age 65 and older (including approximately 18,000 metastatic colorectal cancer patients in this age group diagnosed each year), the partial EVPI just from the Medicare perspective was $13.8 million. The total EVPI was $21.4 million, or $13.4 million from the Medicare perspective, which was slightly lower than the partial EVPI. The higher partial EVPI is likely due to the large and important effect of the efficacy variable in the model, which is not fully reflected in the total EVPI because uncertainty in other model parameters is correlated with uncertainty in the efficacy parameter and works in the opposite direction.

Discussion

Pharmacogenetic testing has the potential to fundamentally alter risk-benefit tradeoffs associated with pharmacotherapy, introducing “personalized medicine” concepts into clinical practice. Cost-effectiveness analyses of pharmacogenetic tests can provide important insights into the relative economic value of this new management paradigm, but few of these cost-effectiveness studies have been published.^{13, 44} This is the first cost-effectiveness study of UGT1A1*28 testing to guide chemotherapy dosing of irinotecan. Testing itself would cost up to $3 million per year, given the test cost and eligible population. We found that incorporating UGT1A1*28 testing in the clinical management of metastatic colorectal cancer treated with irinotecan may result in lower overall medical costs and higher quality-adjusted life expectancy, but our findings were sensitive to several uncertainties in the model’s parameter estimates as indicated by the sensitivity analyses. In particular, the maintenance of clinical efficacy of irinotecan after dose reduction in UGT1A1*28 homozygotes was a key determinant of cost-effectiveness results. That is, if treatment efficacy is not fully maintained after the FDA-recommended dose reduction of irinotecan, testing will not be a cost-effective alternative. The small reduction in treatment-related risks (severe neutropenia and/or death) gained by testing is outweighed by the risk of shortened survival due to under-treatment. However, limited data in this area do suggest that homozygotes experience slower glucuronidation and a greater plasma concentration of SN-38,⁶ and that response rates are not different between homozygotes, heterozygotes, or wild types, even with differential dose reductions across groups.¹⁷

Our analysis indicates that the federal government should be willing to invest in further research to reduce the uncertainty associated with UGT1A1 genotype testing and irinotecan dose-reduction. Specifically, at a WTP threshold of $100,000/QALY and with a 5-year time horizon for the genetic testing technology, the federal government should consider investing at least $13.8 million specifically to evaluate the efficacy of a reduced irinotecan dose in UGTA1*28 homozygous colorectal cancer patients ages 65 and older. In comparison, the estimated expected cost to conduct an average Phase III clinical trial is $27.1 million (in 2000 US$),⁴⁵ although some analysts consider this to be a 2-4-fold overestimate.^{46, 47} The cost to conduct a clinical trial in homozygotes may differ substantially from this amount, however, because irinotecan is not a novel therapeutic entity and already has been proven effective in metastatic colorectal cancer patients. Alternatively, an observational clinical study could be considered that might provide valuable information more quickly.

This analysis is subject to limitations. First, the probabilities of severe neutropenia and death from neutropenia in colorectal cancer patients were difficult to estimate, because data were drawn from randomized controlled trials using different regimens, patients were more closely monitored in clinical trials than in usual care (with dose reductions for patients with adverse events), and the number of events or deaths in any one trial was small or nil. Because we used estimates drawn from clinical trials, our results may under-estimate the value of genetic testing to avoid these events. On the other hand, dose reductions reported in some studies were greater than the dose reduction we modeled based on the FDA label. Results were consistent, however, when we varied the amount of dose reduction benefit from avoiding adverse events. Second, there were only two published clinical trials reporting results in a relatively small number of genotyped patients. So, while genetic test sensitivity and specificity are fairly well established, estimates of clinical sensitivity and specificity – or the probabilities that a positive test predicts an adverse event or death – have a wide range. Although we accounted for the fact that a patient with severe neutropenia could be hospitalized versus treated on an outpatient basis, this is likely to depend on local care patterns and patient ability to self-manage at home. Varying this parameter, however, did not affect the model results.

We also did not consider alternative dosing approaches. For instance, the results of a study by Toffoli and colleagues⁸ indicated that successfully administering full-dose irinotecan in UGT1A1*28 homozygotes can produce superior response rates compared to non-homozygotes (OR 0.19, 95%CI: 0.04–0.89), albeit with higher rates of hematologic and non-hematologic toxicities (OR 4.9, 95%CI: 1.36–17.9). This suggests that the magnitude of the recommended dose reduction in homozygotes should be carefully considered, as a potential effectiveness advantage (i.e., improved response rate and/or survival) may be significantly compromised. In addition, alternative dosing regimens of irinotecan may have different safety profiles, with the importance of UGT1A1*28 status enhanced or reduced depending on pharmacodynamic parameters associated with the dosing schedule. Finally, prophylactic use of agents such as pegfilgrastim or filgrastim could help reduce the incidence of neutropenia in homozygotes,⁴⁸ thereby allowing a full dose of irinotecan to be used in the first cycle, but also contributing to the cost of this treatment strategy.

Our analysis focused on the use of irinotecan for the treatment of metastatic colorectal cancer. The broad therapeutic application of irinotecan (e.g. tumors of the upper gastrointestinal tract and central nervous system^49–51) should also be taken into account when considering both the clinical impact of dosing recommendations and the potential economic value of pharmacogenetic testing. The proportion of metastatic colorectal cancer patients who will eventually receive irinotecan is not known; however, if fewer than 29,214 cancer patients were expected to receive irinotecan in a year, the value of future research would decrease.

Cost-effectiveness analysis can help policy makers evaluate the clinical utility of new medical technologies such as pharmacogenetic tests, make decisions about reimbursement, and identify priorities and investment levels for further research. In the case of UGT1A1*28 testing, we found that routine testing may slightly improve clinical outcomes and quality of life, but is only cost saving if clinical efficacy (survival) is maintained following an irinotecan dose reduction in homozygotes. Further studies to evaluate the impact of irinotecan dose reduction on clinical efficacy would be worth an investment of up to $13.8 million dollars, based on the impact of such testing on costs, life expectancy, and quality of life for Medicare beneficiaries alone, and even more if all use of irinotecan were considered.