Pretherapy Ferumoxytol-enhanced MRI to Predict Response to Liposomal Irinotecan in Metastatic Breast Cancer
To investigate ferumoxytol (FMX)-enhanced MRI as a pretreatment predictor of response to liposomal irinotecan (nal-IRI) for thoracoabdominal and brain metastases in women with metastatic breast cancer (mBC).
Materials and Methods
In this phase 1 expansion trial (ClinicalTrials.gov identifier, NCT01770353; 27 participants), 49 thoracoabdominal (19 participants; mean age, 48 years ± 11 [SD]) and 19 brain (seven participants; mean age, 54 years ± 8) metastases were analyzed on MR images acquired before, 1–4 hours after, and 16–24 hours after FMX administration. In thoracoabdominal metastases, tumor transverse relaxation rate (R*2) was normalized to the mean R*2 in the spleen (rR*2), and the tumor histogram metric rR*2,N, representing the average of rR*2 in voxels above the nth percentile, was computed. In brain metastases, a novel compartmentation index was derived by applying the MRI signal equation to phantom-calibrated coregistered FMX-enhanced MRI brain scans acquired before, 1–4 hours after, and 16–24 hours after FMX administration. The fraction of voxels with an FMX compartmentation index greater than 1 was computed over the whole tumor (FCIGT1) and from voxels above the 90th percentile R*2 (FCIGT1 R*2,90).
rR*2,90 computed from pretherapy MRI performed 16–24 hours after FMX administration, without reference to calibration phantoms, predicted response to nal-IRI in thoracoabdominal metastases (accuracy, 74%). rR*2,90 performance was robust to the inclusion of some peritumoral tissue within the tumor region of interest. FCIGT1 R*2,90 provided 79% accuracy on cross-validation in prediction of response in brain metastases.
This first in-human study focused on mBC suggests that FMX-enhanced MRI biologic markers can be useful for pretherapy prediction of response to nal-IRI in patients with mBC.
Keywords: MRI Contrast Agent, MRI, Breast, Head/Neck, Tumor Response, Experimental Investigations, Brain/Brain Stem
Clinical trial registration no. NCT01770353
Published under a CC BY 4.0 license.
See also commentary by Daldrup-Link in this issue.
In a phase 1 clinical study of participants with metastatic breast cancer, quantitative metrics derived from pretreatment ferumoxytol-enhanced MRI were predictive of tumor response to liposomal irinotecan.
■ In thoracoabdominal metastatic breast cancer metastases, tumor transverse relaxation rate (R*2) relative to spleen relaxation rate (rR*2) computed on a single 16- to 24-hour postferumoxytol (FMX)-enhanced MRI scan predicted response to liposomal irinotecan (nal-IRI) at the tumor level with 74% accuracy.
■ In brain metastases, the fraction of voxels with FMX compartmentation index greater than 1 was computed over the whole tumor (FCIGT1) and from voxels above the 90th percentile R*2 (FCIGT1R*2,90), novel compartmentation index metrics based on the spoiled gradient-recalled echo signal equation, and FCIGT1 R*2,90 predicted response to nal-IRI at the tumor level with 79% accuracy.
Breast cancer is the most diagnosed cancer (1) and the leading cause of cancer-related death in women worldwide (2). Despite continued improvements in available treatments, the 5-year survival rate in those with metastatic breast cancer (mBC) remains low at 27% (3), and treatment options are typically palliative and are limited to hormonal therapy, radiation therapy, and chemotherapy to control symptoms and prolong survival. Irinotecan (IRI) is a topoisomerase I inhibitor used as a salvage treatment in heavily pretreated patients with mBC (4). Liposomal IRI (ONIVYDE; Ipsen Biopharmaceuticals; historic names include nal-IRI [liposomal irinotecan], MM-398, and PEP02) is a liposome-encapsulated IRI with an individual liposome diameter of 110 nm (5). Preclinical studies in models of spontaneous and brain metastases of triple-negative breast cancer showed the accumulation of nal-IRI in metastases. The treatment provided a significant survival benefit at lower doses compared with standard IRI (6,7). In an early-phase study (ClinicalTrials.gov identifier, NCT01770353) of heavily pretreated participants with mBC, the objective response rate to nal-IRI (50 or 70 mg/m2 free base every 2 weeks) was 30% for those with central nervous system disease and 34.5% for those with non–central nervous system disease (3).
Nal-IRI increases the nominal plasma half-life of IRI, permitting the slow deposition of the drug into tumor lesions across leaky vasculature. Nal-IRI is trapped in the tumor extracellular space via the enhanced permeability and retention effect (8), where it is phagocytosed by local macrophages that act as drug depots and release IRI locally (9). The enhanced permeability and retention effect depends on factors such as local tumor-associated macrophage (TAM) content and vascular permeability (10) and physiochemical properties of the nanoparticles (11). The TAMs modulate mechanistic parameters, such as vascular permeability, facilitating the deposition of nal-IRI nanoparticles at the tumor site (12). Across the blood-brain barrier, vascular permeability alone could be an important factor in nanoparticle deposition (13,14). Furthermore, 20–200-nm nanoparticles have desirable tumor accumulation and penetration properties (15). Correlates of nal-IRI deposition would be useful when predicting tumor response to treatment.
While biopsy- or surgery-based pathologic analysis remains the reference standard to monitor tumor response, imaging offers the potential to noninvasively sample the entire volume of multiple tumors in a patient, and imaging using nanoparticle tracers or contrast agents has also been investigated (16,17). Ultrasmall superparamagnetic iron oxides (USPIOs) are nanoparticle contrast agents that induce dose-dependent enhancements in tissue MRI relaxation properties (R1 and R*2). The USPIOs have a unique dynamic distribution process between the vascular and extravascular compartments, resulting in varied MRI enhancement. During the first few hours after infusion, USPIOs reside primarily in the vascular space, with MRI enhancement from the vascular component alone. After approximately 24 hours, as a function of local microvascular permeability, most of the administered USPIOs extravasate (enhanced permeability and retention effect) and are phagocytosed by local macrophages, resulting in an additional delayed MRI enhancement (18,19). Furthermore, the pharmacokinetic and biodistribution characteristics of USPIOs mimic those of nal-IRI (16) and similarly sized nanotherapeutics (17). Therefore, USPIO-enhanced MRI is being investigated as an indicator of tumor microvascular permeability and TAM content.
Ferumoxytol (FMX) (Feraheme; AMAG Pharmaceuticals) is a USPIO approved by the U.S. Food and Drug Administration to treat iron deficiency anemia, and it is used as an off-label MRI contrast agent (14). It is a neutrally charged USPIO with a colloidal particle size of 23 nm (20,21). Preclinical studies showed that FMX colocalized with nanotherapeutics nal-IRI and poly (d,l-lactic-co-glycolic acid)-b-polyethylene glycol within the tumor microenvironment without affecting the pharmacokinetics, biodistribution, or cellular distribution of the nanotherapeutics (16,17). These studies also showed that R*2 enhancement on FMX-enhanced MR images was predictive of treatment response and accumulation of nanotherapeutics at the tumor site (14,16,17,22).
To build on these preclinical studies, we investigated the use of FMX-enhanced MRI as a companion diagnostic tool in a pilot phase 1 study of nal-IRI in advanced solid tumors. In a preliminary study, we observed the colocalization of FMX with macrophages in biopsy tissue from tumors and found that FMX concentration levels computed using post-FMX R*2 were highly correlated with lesion size change in solid tumors (23). A correlation between tumor R*2 measured after FMX and TAMs has also been reported in the setting of pediatric lymphoma and osteosarcoma by Aghighi and colleagues (24) and in the setting of adult glioma by Iv et al (25).
Systemically injected nanoparticles such as nal-IRI and FMX that extravasate into solid tumors may remain extravascular and extracellular in the tumor microenvironment, or they may be compartmentalized inside phagocytic cells, such as TAMs. The success of nanoparticle-based drug therapy depends on individual lesion vascular permeability to the formulation to allow the deposition of efficacious drug concentrations to the patient’s tumor lesions (26–28). Heterogeneity in permeability is observed within and between the lesions (23,28,29). In this phase 1 expansion study in participants with advanced breast cancer treated with nal-IRI, we investigated tumor FMX-enhanced MRI parameters as pretreatment predictors of individual tumor response to nal-IRI. We hypothesized that FMX-enhanced MRI parameters capture individual lesion permeability characteristics for nanoparticles and specifically measure FMX deposition in each tumor lesion; thus, these parameters may be biologic markers of nal-IRI uptake into that tumor that can be used to predict tumor response to nal-IRI. In this report, we apply novel data analysis and fitting strategies to improve the lesion-specific FMX-enhanced MRI signal estimates and describe pretherapy predictors of mBC tumor response to nal-IRI.
Materials and Methods
Ipsen sponsored this prospective study (November 2012–October 2018), and one of the authors (S.G.K.) is an Ipsen employee. All authors, regardless of employment, had control of data inclusion and analysis. This Health Insurance Portability and Accountability Act–compliant study was approved by the institutional review board of all participating institutions, and all study participants provided written informed consent before participating in the FMX-enhanced MRI and treatment portions of the study. Of the 30 participants enrolled, 29 participants with mBC received at least one dose of nal-IRI in the expansion phase of the parent phase 1 open-label nonrandomized study, of whom 27 also had FMX-enhanced MR images that were suitable for analysis. The safety and efficacy results of this expansion study have been published separately (3).
Study Inclusion and Exclusion Criteria
Women aged 18 years or older were enrolled in the study if they had advanced or metastatic breast cancer that had progressed on at least one but no more than five prior regimens in the metastatic setting; Eastern Cooperative Oncology Group performance status of 0, 1, or 2; and acceptable kidney, bone marrow, and liver function. Participants were enrolled in three cohorts (cohort 1, hormone receptor positive and human epidermal growth factor receptor 2 negative; cohort 2, triple-negative breast cancer; cohort 3, breast cancer with active brain metastasis). Notable exclusion criteria included prior IRI or bevacizumab therapy within the preceding 6 months, known hypersensitivity to IRI or FMX, inability to undergo MRI, and administration of parenteral iron in the preceding 4 weeks.
Participants underwent thoracoabdominal and/or brain MRI based on metastasis sites (Table 1). The study schema with timing of FMX-enhanced MRI and standard diagnostic scans is shown in Figure 1. As previously reported, non–central nervous system tumors were assessed using Response Evaluation Criteria in Solid Tumors, version 1.1 (30), with CT or MRI, while central nervous system disease was assessed using modified Response Evaluation Criteria in Solid Tumors (3). MRI scans were acquired before FMX (day 1) and 16–24 hours after FMX (day 2) administration in all participants and at 1–4 hours after FMX (day 1) administration in a subset of participants (n = 18). In six participants, the pre-FMX MRI scans were replicated twice in the same session to assess reproducibility. Ferumoxytol was administered to participants at a dose of 5 mg of iron per kilogram of body weight as an intravenous infusion over 15 minutes, not exceeding 510 mg of iron total. Participants were observed for signs or symptoms of hypersensitivity reactions; this included monitoring vital signs (blood pressure, respiratory rate, and pulse rate) during and for at least 30 minutes after FMX infusion. Within 7 days after FMX infusion, participants received their first dose of intravenous nal-IRI (70 mg/m2 free base), infused over 90 minutes. The protocol was amended on November 3, 2016, to reduce the starting dose to 50 mg/m2 free base with allowance for escalation to 70 mg/m2 free base.
FMX-enhanced MRI Acquisition
Ferumoxytol-MRI data were acquired with 1.5-T General Electric (26 participants) and 3.0-T Siemens (one participant) scanners at six different clinical sites. A series of six fat-suppressed fast spoiled gradient-echo images were acquired with the 1.5- or 3.0-T MRI scanners, respectively, with the following echo times: 1.5 and 2.2, 3.0 and 4.5, 4.5 and 6.8, 6.0 and 9.0, 9.0 and 11.3, and 13.2 and 13.5 msec, as previously described (23). Briefly, the section thickness was 6 mm (1-mm section gap) for thoracoabdominal metastases and 5 mm (no section gap) for brain metastases, and matrix size was 256 × 256 with a field of view to match the size of the imaging section with phantom tubes containing known concentrations of FMX (Fig S1).
Tumors were identified and manually contoured on all applicable sections using mint Lesion, version 3.2 (Mint Medical) by an expert oncoradiologist (R.L.K.) with more than 25 years of experience; tumors were contoured without foreknowledge of eventual responder or nonresponder status of individual lesions. Regions of interest (ROIs) were also manually defined within multiple normal tissues and organs by an expert imaging scientist (N.R.) with more than 20 years of experience. Reference tissues analyzed included muscle, fat, spleen, renal cortex, white matter, cerebrospinal fluid, gray matter, and major blood vessels.
For each participant and imaging session, multiecho FMX-enhanced MR images were preprocessed using in-house Matlab scripts as follows. First, all images were checked for quality and adherence to the study imaging protocol. Next, the multiecho images were spatially coregistered to the reference echo image on which the radiologist drew tumor contours (usually the longest echo time). Globally registered images were then locally registered in regions that contained the contoured tumors.
R*2 mapping.— The signal (S) of a spoiled gradient-echo sequence was modeled using the following equation:
Here, M0, TR, TE, and α are the proton density, repetition time, echo time, and flip angle, respectively. R1 and R*2 are longitudinal and transverse relaxation rates, respectively. Maps of R*2 were computed from pixel intensities of registered multiple-echo-time data fitted using log-linear regression shown by Equation 2: where S and S0 = are the signal at echo time ≠ 0 msec and echo time = 0 msec, respectively.
Estimation of FMXc.— The pixelwise concentration of FMX in brain metastases, FMXc,brain, at the post-FMX imaging session was computed using Equation 3 since these images were coregistered between the pre- and post-FMX sessions:
Quantification of FMX compartmentation (brain metastases only).— The longitudinal relaxivity, r1, of USPIOs would be expected to either not change or decrease due to compartmentation within cells, while the transverse relaxivity, r*2, would be expected to increase (31). Estimation of pixelwise apparent concentration FMXc using coregistered R1 and R*2 maps acquired before and after FMX would therefore theoretically allow for identification of pixels with high concentrations of compartmentalized (phagocytosed) FMX. In our study, we did not acquire R1 maps to limit the total scanning duration for reasons of participant comfort. Instead, for each participant imaging data set, we used a modified spoiled gradient-recalled echo signal equation to calculate the R1-weighted post-FMX image that would be expected for a given FMXc,brain and compared that with the R1-weighted component that was computed from the shortest-echo-time post-FMX image acquired. We did this because pixels with substantial compartmentation of FMX would be brighter on the expected R1-weighted image relative to the computed R1-weighted image. This approach is described in Equations 4–10 and Figure S2, as follows:
Here, r1,FMX is 19 mM−1 sec−1 at 1.5 T and 9.5 mM−1 sec−1 at 3.0 T (32), and R1,0 is the tissue longitudinal relaxation rate in the absence of a contrast agent. The R1-weighted component of the spoiled gradient-recalled echo signal in Equation 4 was computed in Equation 6: where the weighting factor exp(−TE · R*2) · sin (α) accounts for R*2 contribution to the computed signal from field strength, field inhomogeneity, or FMX accumulation. The subscript “i” in Equations 4–6 corresponds to the imaging sessions (see Fig S2), where i = 0, 1, and 2 represents before FMX, 1–4 hours after FMX, and 16–24 hours after FMX, respectively. R*2,i is transverse relaxation rate of the imaging session i. By combining Equations 4, 5, and 6, the signal equation can be written in terms of R1 as in Equation 7:
M0 and r1,FMX represent the pixelwise apparent proton density and longitudinal relaxivity of FMX, respectively.
This model required one pre-FMX scan, an early-phase post-FMX scan, and a delayed-phase post-FMX MRI scan to identify voxels with FMX compartmentation within the cells. A nonlinear regression was used to fit for the parameters R1,0 and M0 in Equation 7. The fitted parameters were then used to calculate fitted R1, and model-expected R1 weighted signal, as shown in Equations 8 and 9:
SExpected,i and SComputed,i represent the model-expected and model-estimated R1 signal, respectively. A compartmentation index was computed as the ratio of SExpected,i and SComputed,i, as shown in Equation 10: values above unity would indicate some degree of extravascular intracellular compartmentation:
Scanner receiver gain differences between the imaging sessions were accounted for by linear scaling of pixel intensities to the mean intensities of the two phantom tubes containing 20 and 80 mg/mL iron that were imaged with each participant. The scaling was applied before the nonlinear regression step.
Responding tumor (RT) and nonresponding tumor (NRT) classes were defined on the basis of maximum percentage diameter change at follow-up relative to baseline. Radiologic diameter change plots for all tumors are shown in Figures 2 and 3. A thoracoabdominal metastasis was labeled RT if it exhibited a diameter decrease greater than 30% (33) and was labeled NRT otherwise. We stratified thoracoabdominal tumors by diameter at baseline and manually pair-matched tumors to create two balanced classes (Table 2). A brain metastasis was classified as RT if it exhibited a diameter decrease greater than 30% and did not progress (an increase from nadir greater than 20% ) and was classified as NRT otherwise (Table 3). In Figures 2 and 3 and Tables 2 and 3, the maximum tumor diameter change is marked as zero if a tumor either did not change in diameter or grew beyond baseline diameter.
FMX-enhanced MRI discriminators.— For thoracoabdominal tumors, relative R*2 (rR*2) per pixel was computed by dividing R*2 by the corresponding spleen ROI average values. Histograms of rR*2 for each tumor were analyzed (details in Fig S3), and the average values of all pixels above the nth percentile were labeled as rR*2,N. For example, the mean value of rR*2 in pixels above the 90th percentile is denoted rR*2,90.
For brain metastases, compartmentation index maps were computed using Equation 10. Participants who underwent brain MRI did not always have accompanying spleen images. Therefore, we only computed unnormalized R*2,90 for brain metastases. From the compartmentation index maps, we computed two metrics for each brain metastasis: (a) the fraction of pixels within a tumor volume of interest with compartmentation index greater than 1 (FCIGT1) and (b) FCIGT1 and R*2 in the 90th and higher percentile (FCIGT1R* 2,90). The area under the receiver operating characteristic (ROC) curve (AUC) for discrimination of RT and NRT tumors was computed for both metrics.
Optimizing the FMX-enhanced MRI Discriminators for Classifying Thoracoabdominal Tumors
To classify thoracoabdominal tumors as RT or NRT, we optimized the threshold value for rR* 2. We investigated both the optimal percentile (nth bin) and rR* 2,N as follows.
Optimal percentile bin selection.— For rR*2, mean values of all pixels in the nth bin were calculated for the 5th–99th percentile, and AUC was computed for rR*2,5 through rR*2,99. The robustness of the AUC for rR*2,N at each N to variability in the tumor ROI definition was assessed using Reproducibilityindex,N (details in Appendix S1). An optimal value of N (Nopt) was chosen that provided both high AUC for discrimination between RT and NRT tumor classes and a high reproducibility index (Equation S4 in Appendix S1) with respect to ROI dilation and erosion. The AUC was reported as AUC ± SE·Zcrit (details in Appendix S2).
Optimal cutoff value rR*2,N opt.— For the identified optimal percentile bin, Nopt, ROC analysis was performed to identify optimal cutoff value for the discriminator rR*2,N opt using the distance-to-corner criterion (34). The ROC plots were generated using R software, version 4.1.3 (R Project for Statistical Computing).
Repeatability and Generalizability
Bland-Altman analysis was used to assess the repeatability of R*2 measurements in a reference tissue, namely muscle. For each participant, the R*2 average across multiple repeated sessions is plotted against the R*2 difference. The coefficient of repeatability (35), which measures the maximum difference likely to occur between repeat measurements, was computed using Equation 11: where n is the number of participants and di,1 and di,2 are repeated measurements of participant i.
The coefficient of variation was calculated using Equation 12:
To test the local generalizability of the discriminators, we performed leave-one-out cross validation (LOOCV). The performance of the LOOCV test was quantified by calculating the number of correctly identified tumors divided by the number of possible permutations of LOOCV runs.
The significance of observed differences between pre-FMX and post-FMX values of R*2 in different tissues was assessed using a two-sample t test in Matlab, version 2018b (MathWorks). In addition, changes in R*2 after FMX injection were compared across thoracoabdominal and brain metastasis. Bonferroni correction for multiple comparisons was applied, resulting in a significance level of P < .001. Mean R*2 (rR*2) between RT and NRT in thoracoabdominal metastases was compared using the t test, with P < .01 considered to indicate a significant difference. Violin plots (36) were used to visualize the differences in R*2 (rR*2) between RT and NRT.
FMX-enhanced MRI Participant Characteristics
Participant demographics are summarized in Table 1. FMX-enhanced MRI studies from a total of 27 (mean age, 52 years ± 10 [SD]) participants were analyzable: 17 participants (mean age, 54 years ± 8) underwent only thoracoabdominal FMX-enhanced MRI, six participants (mean age, 47 years ± 11) underwent only brain FMX-enhanced MRI, and the remaining four participants (mean age, 49 years ± 7) underwent both brain and thoracoabdominal FMX-enhanced MRI. Two participants each from the thoracoabdominal and brain metastasis arms were excluded from the analysis because of a lack of clinical follow-up information on tumor size. In addition, one participant from the brain metastasis arm was excluded from analysis due to poor image quality of acquired multiecho data. A total of 49 thoracoabdominal (19 participants; mean age, 48 years ± 11) (liver, n = 37; lymph node, n = 3; peritoneum, n = 3; pleura, n = 2; chest wall, n = 2; pancreas, n = 1; retroperitoneum, n = 1) and 19 brain (seven participants; mean age, 54 years ± 8) metastases were analyzed.
Analysis of Tissue R*2 Changes 16–24 Hours after FMX versus before FMX
Figure 4A and 4B shows example R*2 maps computed before FMX and 24 hours after FMX in a participant with liver metastases. Qualitatively, a significant increase in R*2 can be observed in organs of the mononuclear phagocyte system (liver and spleen), blood vessels, and kidney. Tumor R*2 also appeared heterogeneously higher after FMX relative to before FMX. Figures 4E and 4F depict R*2 maps computed before FMX and 24 hours after FMX in a participant with brain metastases. R*2 in tumor and blood vessels was higher after FMX compared with before FMX, while R*2 changes in normal brain parenchyma were relatively small. Quantitative R*2 values in various tissue types are presented in Figure 5A. As expected, R*2 of tumors, mononuclear phagocyte system, blood vessels, and the kidney was higher (P < .001) after FMX than before FMX. The post-FMX versus pre-FMX increase in R*2 of thoracoabdominal metastases (ΔR*2 = 83 sec−1) was more than twice (P < .001) the increase observed in brain metastases (ΔR*2 = 33 sec−1).
In six participants, we obtained repeat studies in the pre-FMX session to assess the repeatability of R*2 measurements. The Bland-Altman repeatability test on the pre-FMX reference muscle tissue in these six participants is shown in Figure 5B. All calculated difference points are within the 95% CI line. The coefficient of repeatability was 6.36 sec−1, and the coefficient of variation was 6.5%.
Tumor R*2 Normalization Improves Classification of Thoracoabdominal Metastases
Of the systemic tumors assessed with Response Evaluation Criteria in Solid Tumors, most lesions (28 of 49 [57%]) experienced a decrease in diameter after therapy (Fig 2). To control for the effect of pretreatment tumor diameter in response to nal-IRI treatment, we manually matched the pretreatment diameters of tumors and created two balanced classes of RT (21) and NRT (21) for our analysis (Table 2). While the 16- to 24-hour post-FMX mean R*2 of the systemic whole tumors showed no evidence of a difference between the two classes (Fig 6A), the spleen-normalized mean rR* 2 of whole tumors at 16–24 hours after FMX was different between the two classes (P < .01, Fig 6B), with an AUC of 70% (Fig S5).
Ninetieth Percentile for Classification of Thoracoabdominal Metastases
The AUC of rR*2,N for discrimination between NRT and RT was found to increase with N (Fig 6D and Fig S5). However, the Reproducibility index in tumor rR* 2,N with erosion and dilation of the tumor ROI was lower for higher N (Fig 6E). Thus, accuracy and reproducibility moved in opposite directions as N was increased from the 80th to the 99th percentile. We selected a compromise value of Nopt = 90th percentile, which provided good classification performance (AUC) and acceptably low sensitivity to variability in the tumor contours (higher Reproducibility index).
Thoracoabdominal Metastases Response Prediction Using rR*2,90
Complete ROC plots of rR*2,90 and R*2,90 are shown in Figure S6. Normalization of R*2 values to the respective mean values in the spleen improved the AUC. To discriminate between RT and NRT, the AUC was 0.77 ± 0.1 (95% CI) for rR*2,90 (Table 4, Fig S6). The optimal threshold value of rR* 2,90 was 1.07. For the optimum threshold, training accuracy and LOOCV accuracy were 76% and 74%, respectively (Table 4).
Thoracoabdominal Metastases: Classification Accuracy Is Sensitive to ROI Erosion
Radiologist-drawn tumor contours were eroded and dilated by 1 to 5 pixels in all directions in plane to investigate the impact of tumor contouring variability on performance of the tumor classifier rR*2,90. As observed in Figure 7A, the AUC of the rR*2,90 classifier decreases monotonically as the radiologist-drawn tumor contours are eroded by 1 to 5 pixels. In contrast, dilation of the radiologist-drawn contours by 1 to 5 pixels did not affect the AUC of rR*2,90 (Fig 7B).
Brain Metastases: Distribution of Voxels with Compartmentation Index of 1 or Greater Correlates with Response
We computed compartmentation index maps as surrogates for mismatch between the expected R1 enhancement for a given R*2 enhancement. Figure 8A shows the computed and expected R1-weighted maps, compartmentation index map, and R*2 maps for three example tumors. Qualitatively, on the compartmentation index maps, the responding tumor had a higher distribution of FCIGT1 voxels than a tumor that initially responded but regrew and a progressing tumor. The difference was less clear on R*2 maps, with a responding tumor and a tumor that first responded then progressed showing high values compared with the tumor that progressed immediately after treatment. The ROC curves for the three putative discriminators are plotted in Figure 8B, and the corresponding AUC values are tabulated in Table 4. R*2,90 provided an AUC below 0.5, indicating poor classification power. However, the compartmentation index metrics enabled good discrimination of RT versus NRT tumors, with AUCs of 0.75 and 0.86 for FCIGT1 and FCIGT1R*2,90, respectively. FCIGT1 R*2,90 had good accuracy on both training (84%) and LOOCV (79%).
FMX-enhanced MRI has been shown to correlate with macrophage content in tumors (23–25), which may be mechanistically relevant, as TAMs can affect vascular permeability, and thus, deposition of nal-IRI (37); may alter its phagocytosis; may serve as a drug depot; and may contribute to payload release with subsequent conversion of IRI to its active metabolite, SN-38 (9). In a phase 1 expansion study of participants with mBC, we investigated FMX-enhanced MRI as a pretreatment predictor of response to nal-IRI in both thoracoabdominal and brain metastases. The post-FMX versus pre-FMX increase in R*2 of thoracoabdominal tumors (ΔR*2 = 83 sec−1) was more than twice (P < .001) the increase observed in brain metastases (ΔR*2 = 33 sec−1), indicative of lower microvascular permeability in brain metastases compared with thoracoabdominal metastases in participants with mBC. Therefore, we assessed brain and extracranial metastases with different methods. From MRI performed 16–24 hours after FMX, we computed tumor R*2 normalized to spleen R*2 in thoracoabdominal metastases. A mean pretreatment spleen-normalized R*2 of tumor voxels in the 90th percentile or higher (rR*2,90) higher than 1.07 was predictive of response to nal-IRI with a mean AUC of 0.77 ± 0.1 (95% CI) and an LOOCV of 74%. In brain metastases, we computed a measure of intracellular compartmentation of FMX, which may result from TAM phagocytosis of the FMX, which was predictive of the response of brain metastases to nal-IRI. The FCIGT1R*2,90 metric provided 79% accuracy on LOOCV for discriminating between RT and NRT brain metastases, with RT having higher pretreatment values of FCFGT1R*2,90 than NRT.
In thoracoabdominal metastases, unnormalized R*2 was not useful in the prediction of RT versus NRT in this study. Normalization of tumor R*2 to the mean R*2 in the spleen (rR*2) increased the predictive power for distinguishing between RT and NRT. Spleen R*2 was chosen as a reference for normalization because together with the liver, the spleen is a major reservoir of phagocytic cells and accumulates nanoparticles and liposomes, thus affecting relevant pharmacokinetics of FMX and nal-IRI in the systemic circulation and deposition characteristics into other tissues, including tumors. Although all participants received a fixed dose of 5 mg of iron per kilogram of body weight FMX (up to 510 mg of iron as a maximum dose), normalization of tumor R*2 to spleen R*2 measured in the same patient and imaging session will compensate for small patient-to-patient differences in FMX pharmacokinetics due to differences in blood volume (38) and activity of the mononuclear phagocyte system (39). A previously reported approach for quantification of FMX uptake into tissue requires both a calibration phantom and baseline pre-FMX scans for reference (23). In this study, we have investigated the feasibility of obviating the need for a pre-FMX scan on the hypothesis that R*2 values above some threshold are likely to reflect FMX accumulation rather than intrinsic magnetic susceptibility. Importantly, measurement of rR*2,90 requires only one FMX-enhanced MRI scan, performed 16–24 hours after administration of FMX, and does not require calibration of R*2 values against an FMX phantom.
rR*2,90 is a reporter of combined passive and phagocytic accumulation of FMX nanoparticles in the tumor. An FMX-enhanced MRI biologic marker of pretreatment TAM content may be mechanistically predictive of a given response of the tumor to nal-IRI. We have sought to compute an index of FMX compartmentation by exploiting the fact that compartmentation within cells will have opposing effects on R1 and R*2 FMX MRI enhancement. In a recent clinical glioblastoma study, Barajas et al (40) used information from susceptibility weighted images acquired at three different phases of FMX distribution to compute segregation and extravascular localization of FMX imaging maps. The positive and negative segregation and extravascular localization of FMX imaging values were correlated with increasing TAM infiltration and increasing microvascular density regions, respectively. The segregation and extravascular localization of FMX imaging–positive metric correlated with the overall survival of the participants with glioblastoma. Computation of the compartmentation metrics requires scans from a pre-FMX and two post-FMX sessions within 24 hours. Additionally, linear scaling against FMX phantom tubes is also required to correct for receiver gain and coil sensitivity differences between the multiple MRI sessions. It is conceivable that the need for additional FMX-enhanced MRI and inclusion of FMX phantoms will be obviated if both R1 and R*2 maps are acquired during the 16- to 24-hour post-FMX session, though this remains to be investigated.
We investigated intrasession repeatability of the R*2 measurement in a subset of six participants. The coefficient of variation was 6.5% for repeat measurements of R*2 in muscle, with excellent agreement over a range of R*2 values with Bland-Altman analysis. Our measurements of R*2 in different tissues before and after FMX showed good agreement with values reported by Stirrat et al (41). We also investigated the sensitivity of the predictive performance of rR*2,90 to perturbations of the radiologist-drawn tumor contours. Interestingly, the metric was relatively insensitive to dilation of the ROIs by 1 to 5 pixels in plane, but its performance was degraded by erosion of the manual ROIs by 1 to 5 pixels in plane. These results may reflect the abundance of FMX nanoparticles located at the tumor periphery. Thus, erring on the side of including more peritumoral tissue within the tumor contours is preferable for tumor response assessment with FMX-enhanced MRI.
This study had some limitations. First, the study sample was relatively small, at 27 participants with 49 thoracoabdominal and 19 brain tumor lesions that were evaluable. Second, reproducibility was tested only for pre-FMX scans, and an initial assessment of generalizability of the pretreatment predictors of response was performed with LOOCV, but prospective testing is required to establish reproducibility and generalizability more rigorously. Third, there were minor participant-to-participant variations in voxel dimensions for a given anatomic site (brain or thoracoabdominal). Spatial resolution can affect R*2 contrast enhancement due to dependence of susceptibility on the underlying geometry. The echo times used in this study were less than 13.5 msec, and published studies indicate that susceptibility contrast changes are relatively insensitive to spatial resolution variations at short echo times (<20 msec) (42). While the spatial resolutions of brain and thoracoabdominal images were significantly different, the impracticality of obtaining spleen images for normalization in participants with only brain metastases meant that independent predictors were developed for metastatic lesions in the two anatomic locations.
In conclusion, we showed pretreatment FMX-enhanced MRI as a marker of response to nal-IRI in participants with mBC with thoracoabdominal and brain metastases. In thoracoabdominal metastases, we showed that rR*2 computed from one 16–24-hour post-FMX-enhanced MRI scan can be used as a pretherapy response predictor; this could simplify clinical implementation and logistical concerns, as calibration phantoms or pre-FMX scans may not be needed. In brain metastases, we computed a novel compartmentation index based on the spoiled gradient-recalled echo signal equation applied to phantom-calibrated FMX-enhanced MRI scans that were acquired before FMX, 1–4 hours after FMX, and 16–24 hours after FMX, which had high accuracy on LOOCV for predicting response in brain metastases. We analyzed only pretherapy response prediction of individual tumor lesions, which may not necessarily equate to overall participant-level prediction, as we did not account for heterogeneity of response within the same participant, nontarget lesions, or appearance of new lesions. Any bearing to patient-level response prediction must be tested in future studies. Furthermore, follow-up testing should include acquisition of both R1 and R*2 maps at the 16–24-hour post-FMX time point to potentially prevent the need for phantom tubes and additional FMX-enhanced MRI time points.
The authors thank all participants involved in the study as well as their caregivers, care teams, investigators, and research staff at participating institutions. We thank Daniel D. Von Hoff, MD, for valuable consultations and advice throughout this study.
Author contributions: Guarantor of integrity of entire study, N.R.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; agrees to ensure any questions related to the work are appropriately resolved, all authors; literature research, H.R., J.R.C., S.G.K., N.R.; clinical studies, J.R.C., H.S.H., D.K.J., J.C.S., R.L.K., N.R.; experimental studies, N.R.; statistical analysis, H.R., A.M.A.L., N.R.; and manuscript editing, H.R., A.M.A.L., J.R.C., D.K.J., S.G.K., J.C.S., R.L.K., N.R.
1 Current address: Biosplice Therapeutics, San Diego, Calif.
This study was sponsored by Ipsen and supported in part by research funding From the National Institutes of Health (P30 CA076292, QI Core).
Data sharing: Where patient data can be anonymized, Ipsen will share all individual participant data that underlie the results reported in this article with qualified researchers who provide a valid research question. Study documents, such as the study protocol and clinical study report, are not always available. Proposals should be submitted to [email protected] and will be assessed by a scientific review board. Data are available beginning 6 months and ending 5 years after publication; after this time, only raw data may be available.
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Article HistoryReceived: Feb 18 2022
Revision requested: Apr 6 2022
Revision received: Nov 10 2022
Accepted: Dec 20 2022
Published online: Feb 03 2023