Early assessment of response to induction therapy in acute myeloid leukemia using ¹⁸F-FLT PET/CT

Acute myeloid leukemia (AML) is a biologically and clinically heterogeneous disease with a distinct response to treatment and clinical outcome. AML treatment typically entails induction chemotherapy aimed at debulking disease and rapidly restoring hematopoiesis, followed by consolidation therapy to eliminate residual disease. Only a few predictive factors are used to make treatment decisions in clinical practice, although the prognosis of patients with AML is influenced by several patient-specific and disease-related risk factors, among which cytogenetic risk is most important [1].

Early assessment of the response to induction therapy is important for defining chemosensitivity and for planning subsequent treatment and can be valuable in predicting prognosis in patients with AML. Previous studies have reported that the early response to induction therapy in AML is an independent predictor of subsequent complete remission (CR) and is associated with the long-term outcome [2, 3]. The current National Comprehensive Cancer Network and European LeukemiaNet guidelines recommend an early bone marrow (BM) evaluation, at 14−21 days after the start of induction therapy, to assess the early response to treatment and to guide further treatment [4]. Recently, however, the interim BM assessment has come into question. Systematic study has never been tested for the value of interim BM assessment. The predictive value of the interim BM assessment for achieving CR varies in sensitivity (40−90%) and specificity (43−79%) [5–8]. Not all patients with negative interim BM results achieve CR, while some patients with significant blast numbers in the interim BM show recovered counts and documented CR without undergoing further therapy. Furthermore, the lack of standardized criteria for the optimal response or residual/refractory disease makes it more challenging to use interim BM results to guide further management. BM aspiration is generally regarded as safe, but invasive [9, 10]. Most patients report pain during BM aspiration, with 36% grading this pain as moderate to severe; this causes stress and anxiety for patients and their families. Additionally, invasive procedures are associated with risks of bleeding and infection in the BM nadir after induction, although the complication rate is low [9, 10].

¹⁸F-fluorodeoxythymidine (¹⁸F-FLT) is a radiopharmaceutical for positron emission tomography (PET) that reflects cell proliferation. ¹⁸F-FLT is trapped after phosphorylation by thymidine kinase1 (TK-1), whose expression is increased in replicating cells [11]. Because ¹⁸F-FLT uptake is specific for cycling cells, ¹⁸F-FLT PET could be used in hematologic diseases featuring BM abnormalities. Agool et al. [12, 13] have demonstrated that ¹⁸F-FLT PET imaging can be used to evaluate and quantify BM proliferation and is useful for distinguishing hematologic disorders. This study aimed to evaluate whether ¹⁸F-FLT PET/computed tomography (CT) is suitable for assessment of the early response to induction chemotherapy and is helpful for predicting prognosis in patients with AML.

Although BM aspiration with morphological assessment remains the standard for residual disease assessment in patients with AML, its predictive value for residual disease is suboptimal and the technique is invasive [5–8]. To complement this limitation of BM aspiration, noninvasive modalities have been applied to assess the presence of viable residual cells in BM after induction. Several studies have suggested that dynamic contrast-enhanced magnetic resonance imaging of BM can be used to assess the changes in microvascular density and could be a useful prognostic indicator of disease activity and survival [17–19]. ¹⁸F-FDG PET imaging, which measures glucose metabolism, has also been shown to have potential for evaluation of leukemic BM infiltration [20].

As a surrogate of cellular proliferation, ¹⁸F-FLT PET has been used for early therapeutic monitoring in various cancers [21–24]. The rate-limiting step for intracellular ¹⁸F-FLT retention in proliferating cells is phosphorylation by TK-1, and thus, ¹⁸F-FLT accumulates in proportion to TK-1 activity [25]. A more than 10-fold overexpression of TK-1, which is the key enzyme for intracellular ¹⁸F-FLT accumulation, is observed in leukemic blasts [26]. Only a few studies have reported the feasibility of ¹⁸F-FLT PET imaging in AML. In a pilot study by Buck et al. [27], patients with relapsed, refractory, or untreated leukemia showed higher ¹⁸F-FLT uptake in the BM and spleen than did controls. This indicated that ¹⁸F-FLT PET reflected disease activity, but there was no significant correlation between ¹⁸F-FLT uptake and the number of leukemic blasts identified in BM biopsy. The authors discussed that the proliferative activity of leukemic blasts and normal BM cannot be differentiated using ¹⁸F-FLT PET, as the normal hematopoietic cells also show increased ¹⁸F-FLT uptake. In another study of AML patients who underwent induction therapy, the BM uptake in RD patients was significantly greater than in normal controls, while the BM uptake in CR patients was similar to that in normal controls in pre-treatment ¹⁸F-FLT PET images [15]. That study also investigated whether ¹⁸F-FLT PET during or after induction therapy could be useful for early assessment of the clinical response. All 5 CR patients exhibited low BM uptake, while 2 RD patients displayed elevated BM uptake (maximum SUV, 3.6 ± 0.4 vs. 11.4 ± 0.8, p < 0.001). Thus, the study concluded that ¹⁸F-FLT PET during induction chemotherapy could predict successful BM ablation early on. We presumed that increased ¹⁸F-FLT uptake is associated with the degree of RD in the present study, because all PET/CT scans were performed shortly after myeloablative chemotherapy. Our results indicated that interim ¹⁸F-FLT PET/CT performed well in the early prediction of clinical response to induction therapy in patients with AML. All 5 patients with PET-negative findings achieved CR in the follow-up BM aspiration 4−6 weeks after induction therapy (high negative-predictive value), and both of the RD patients showed PET-positive findings (high sensitivity).

Leukemic distribution can be heterogeneous or localized, despite the malignant systemic disease [28, 29]. Vanderhoek et al. [15] demonstrated substantial heterogeneity of BM uptake in the ¹⁸F-FLT PET response assessment, both during and after induction therapy. The distribution of ¹⁸F-FLT uptake in RD patients was more heterogeneous (higher CV) than that in CR patients. Additionally, 1 RD patient was in an aplastic state at the interim BM biopsy, despite the significantly increased BM uptake in the ¹⁸F-FLT PET image. The BM biopsy is usually performed on the unilateral posterior iliac bone and the BM evaluation is thus limited to that area and cannot reflect the whole of the BM in vivo; therefore, residual leukemia might be missed. ¹⁸F-FLT PET imaging is performed from the vertex to the upper thigh after a single injection, facilitating noninvasive assessment of the total hematopoietic BM compartment, which represents a significant advantage over a BM biopsy.

The present study also suggested that ¹⁸F-FLT PET/CT may perform better than follow-up BM assessment in predicting relapse. Three patients with PET-positive findings achieved CR in the follow-up BM aspiration (false-positive). Of these, 2 patients (patients 6 and 7) had confirmed or clinically suspected relapse within 4 months after consolidation, even though 1 patient had been classified as favorable-risk. On the other hand, patient 3, with relapsed AML, had PET-negative findings and has now been in the third remission for 34 months, although relapse after achieving remission is considered a poor prognostic factor in patients with AML [30]. One of the 5 PET-negative patients relapsed; this patient showed a heterogeneous BM distribution (CV = 0.32). In a study conducted by Vanderhoek et al. [15], the CVs of all patients with residual/refractory disease on the interim BM biopsy were ≥ 3.0. BM heterogeneity as well as visual analysis may be important for predicting relapse. Current risk-adapted therapy in adult patients with AML is based on only a few prognostic markers, such as age, cytogenetic risk, and gene mutations at diagnosis. More recently, adjustment of therapy based on incorporation of post-treatment data is likely to become increasingly important. When added to the current risk-stratification system, ¹⁸F-FLT PET/CT may provide a more accurate selection of patients at high risk for relapse, and thus improve survival by modification of treatment.

In our CR patients, 1 false-positive PET case (patient 9) showed persistent mild thrombocytopenia (Table 6). Agool et al. [12] reported that significantly increased ¹⁸F-FLT uptake was observed in patients with myelodysplasia, myeloproliferative disorder, and myelofibrosis. Although our patient was not further evaluated for thrombocytopenia, there was a possibility that BM dysfunction caused the false-positive PET finding.

The main limitation of this study is the small sample size. Additionally, molecular studies were not available in all patients. Risk stratification, based on combined clinical and molecular markers, has recently been proposed to improve the predictive value of early response assessment [1, 4]. Our results suggest that further studies on the prognostic potential of the combination of molecular abnormalities and interim ¹⁸F-FLT PET/CT results are warranted. The heterogeneity of compliance to the imaging protocol is another potential limitation. We originally planned to standardize the imaging protocol strictly. However, variability in the injected activity and in the time from injection to PET scanning was unavoidable, and some cases had accidental extravasation at the injection site. Poor protocol compliance may lead to over- or underestimation, particularly in terms of quantitative PET analysis. To overcome this limitation, we performed both qualitative and quantitative analyses. In qualitative analysis, we experienced no difficult cases when determining whether the image was positive or negative according to our criteria. For solid cancers that are already frequently and usefully evaluated with PET/CT, qualitative PET analysis is as important as quantitative PET analysis. For lymphoma, the Deauville criteria is based on visual analysis [31]. Lastly, there were neither standards in place regarding the time for the ¹⁸F-FLT PET/CT imaging of patients with AML nor any established criteria for interpreting the images. The role of imaging studies, including PET/CT, is still limited in terms of evaluation of hematologic malignancy. However, application of ¹⁸F-FLT PET/CT to patients with AML has been continuously studied although the number is small [15, 27, 32]. Recently, a phase II clinical trial assessing ¹⁸F-FLT PET/CT in AML was launched [32], and this prospective ECOG-ACRIN EAI141 is likely to provide more conclusive results when completed. Our preliminary results could be of value in planning future clinical studies (e.g., determining appropriate time points for performing ¹⁸F-FLT PET/CT), data analysis (e.g., setting the criteria for visual analysis), and the clinical interpretation of ¹⁸F-FLT PET/CT images in patients with AML. We anticipate that these study results will collectively contribute to the establishment of clinical applications of ¹⁸F-FLT PET/CT imaging in patients with AML.

The results of this preliminary study showed good sensitivity and negative-predictive value for evaluating residual disease in patients with AML by means of ¹⁸F-FLT PET/CT, although unexpected discrepant findings were noted in 3 of 10 cases, possibly reflecting the heterogeneous nature of AML. Our results suggest that ¹⁸F-FLT PET/CT may have value as a noninvasive tool for early assessment of the response to treatment and may provide prognostic value for survival in patients with AML.