Metabolic Response of Glioblastoma to Superselective Intra-Arterial Cerebral Infusion of Bevacizumab: A Proton MR Spectroscopic Imaging Study

Discussion

The inability of conventional MR imaging to permit reliable clinical assessment of tumor progression and therapeutic response has heightened the need for the development of more effective neuroimaging approaches. In this respect, the emergence of alternate modalities has expanded the role of imaging beyond anatomic structural information to enable the analysis of physiologic and metabolic processes that may underlie neoplastic transformation.^{12,14,19⇓⇓–22} Particularly noteworthy is the utility of MRS in complementing conventional MR imaging for assessing treatment efficacy^12,14; MRS is especially attractive in the setting of recurrent GB, given the association of bevacizumab with dramatically reduced nonspecific contrast enhancement. By using MRS to assess neurochemical changes that may be associated with bevacizumab, we hoped to elucidate some aspects of the complex mechanisms that seem to underlie the tumoricidal effects of the intervention. These mechanisms may include inhibition of neovascularization and/or normalization of existing tumor vessels resulting in decreased peritumoral edema.^{4,8⇓⇓–11}

Bevacizumab and other anti-angiogenic agents may result in marked nontumoral decreases in contrast enhancement, termed “pseudoresponse.” This phenomenon is characterized by a posttreatment decrease in the enhancing portion of the lesion, with progression of the nonenhancing portion on T2-weighted sequences, and may be observed as early as 1–2 days after initiation of therapy.¹⁷ Awareness and recognition of this apparent, favorable radiographic pattern of change is critical for appropriately interpreting treatment response change, as the striking reduction of contrast enhancement may partly reflect normalization of the blood-brain barrier and decreased tumoral vessel permeability, rather than a true decrease in tumor volume.^17,23 These anti-angiogenic agents demonstrate marked reductions in contrast enhancement, with high response rates and 6-month progression-free survival, but have rather modest or no overall survival benefits.^17,23 Currently, no particular imaging technique is capable of differentiating between a true response and pseudoresponse.²⁴

Standardized criteria for assessment of treatment response have been published and include the Macdonald criteria from 1990²⁵ and the updated proposed recommendations by the RANO Working Group in 2010.¹⁷ Although the initial Macdonald criteria were based upon CT, these have evolved with application to conventional MR imaging, which has become the standard noninvasive neuroimaging technique for assessment of tumor burden. According to this method that has served as the current benchmark for tumor assessment, evaluation of therapy response is based on the change in contrast-enhancing lesion size via 2D perpendicular measurements of the enhancing region. Clinical factors also taken into consideration include steroid dosage and the neurologic state of the patient. Various important limitations of the Macdonald criteria have since been acknowledged, primarily the nonspecific nature of contrast enhancement reflecting breakdown of the blood-brain barrier resulting from various etiologies; not necessarily representing the equivalent surrogate of tumor burden; and the lack of assessment of the nonenhancing disease component, often representing a combination of nonenhancing neoplasm, edema, and the sequelae of treatment. Recognition of these shortcomings has resulted in updated criteria by the RANO group, which address the nonenhancing areas of T2 hyperintense signal abnormality.¹⁷

In Table 2, we detail the individual response rates for our cohort of 18 patients utilizing the updated RANO response criteria. While there are no comparable studies evaluating early radiographic response utilizing the updated RANO criteria for recurrent GB, the best available data show that median PFS is 4.2 months for IV bevacizumab alone and 5.6 months for IV bevacizumab plus irinotecan.²⁶ Recent data from an internal study at our institution that is in press and includes this phase I patient cohort have demonstrated more encouraging outcomes by using the RANO criteria, with median PFS increasing to approximately 10 months for patients treated with a single IA bevacizumab dose followed by IV bevacizumab delivery.⁷

Still, the inherent inability of routine contrast-enhanced MR imaging to differentiate between response, progression, and/or treatment effects has led to increased interest in metabolic and functional neuroimaging techniques based on MR and PET.²⁷ Sophisticated MR methods now rely upon the hemodynamic properties of gliomas, which include cerebral blood volume, cerebral blood flow, vascular permeability, and blood vessel diameter.²⁷ New PET radiotracers target neoplastic biologic activity, enabling more specific interrogation of tumor physiology with assessment of hypoxia and proliferation rate.²⁸

Proton MRS is an alternative surrogate marker that can assist in monitoring treatment response and in clinical decision making. By allowing direct neurochemical analysis of several potential biomarkers of disease, MRS may serve as an important adjunctive technique.^{12⇓⇓–15} Metabolites of potential clinical interest include the putative neuronal density and viability marker NAA, which is decreased in most brain lesions; the cell bioenergetics marker tCr, whose resonance includes a contribution from phosphocreatine and remains unchanged under most in vivo conditions, aside from extensive tissue damage or disrupted enzymatic homeostasis; the cell membrane biosynthesis and metabolism marker tCho, which increases in cell membrane breakdown and proliferation; and the anaerobic energy metabolism marker Lac, which is increased with anaerobic glycolysis activity.^12,21 Thus, MRS represents a viable noninvasive neuroimaging technique for probing the metabolic disturbances that may elucidate the mechanisms of action of this promising intervention.

Our use of MRS in the present study has demonstrated the potential of the technique, revealing significant and trend-level significant reductions of tCho/NAA ratios in enhancing and nonenhancing regions of disease, respectively. Measurement of changes in tCho has emerged a reliable predictor and indicator of neoplastic response,¹² in concordance with the results of the present study. Our failure to find statistically meaningful changes for the matched contralateral normal parenchyma, as well as for normal white matter control voxels, seems to add face validity to our statistically significant findings in the enhancing and nonenhancing components of disease. While the absence of a significant difference in the matched “normal” and white matter areas was expected, our finding of a significant increase in tCho/NAA ratios within the gray matter ROIs (+8.62%; P = .02) was unexpected. One consideration is that this finding may reflect treatment-related neuronal loss and gliosis occurring to a greater extent compared with treatment effects on the neoplasm. More specifically, this might be seen in the setting of NAA values decreasing to a greater extent relative to decreasing tCho values, which would result in an overall increase in the tCho/NAA ratio. In addition, the statistically significant differences in median tCho/NAA ratios between the enhancing ROIs compared with the control, gray matter, and white matter groups further suggest that GB treatment with SIACI of bevacizumab may be associated with an antiproliferative effect.

Discordant findings were noted in 2 patients, with unexpected increases in the tCho/NAA ratio from the pretreatment to posttreatment scans. Patient 1 demonstrated a 123.94% increase in tCho/NAA in the enhancing ROIs, and patient 5 showed a 37.67% increase in tCho/NAA in the nonenhancing T2 hyperintense voxels (Table 2). Possible contributing factors for these 2 outlier nonresponders include low bevacizumab dosing (both were early patients at the start of the dose escalation Phase I trial), differences in specific tumoral genetics of these patients' neoplasms, and/or prior exposure to IV bevacizumab (in the case of patient 5).

This study has a number of limitations that suggest caution in generalizing the results. First, the sample size of 18 subjects is relatively small, so this report should be viewed as an initial description and interim analysis of our clinical experience to date, which is nevertheless promising. Second, as 1 effect of bevacizumab is to decrease edema by reducing the abnormal permeability of tumor vessels, our observed decrease in tCho/NAA ratios may, in part, reflect the replacement of normal brain tissue into the relevant voxel. Third, clinical heterogeneity among patients selected for inclusion into the phase I/II SIACI of bevacizumab trials warrants consideration. This included variation in prior IV bevacizumab status with 6 of the 18 patients having previous exposure. Ideally, only bevacizumab-treatment-naïve patients should have been included to allow more accurate assessment of the true implications of this potential therapy. Our group is currently seeking funding for a larger controlled prospective study to directly compare IA and IV therapy in bevacizumab-naïve patients with recurrent GB. Similarly, the study sample was heterogeneous with respect to previous steroid exposure, with 5 of the 18 patients being steroid naïve. These factors are potential confounders to clinical outcomes and study results. Furthermore, the number of days from SIACI treatment to the postinfusion MRS study was not uniform, ranging from 15–30 days.

Various technical limitations should also be considered. The subjective component of selecting matching ROIs on the pre- and posttreatment scans, despite interval morphologic changes, introduces sampling error. Thus to some extent, differences in tissue type sampling may partially contribute to the apparent changes in metabolite concentrations. Second, inherent differences in anatomic resolution and difficulty in precisely coregistering the MRS localizer voxels with the corresponding anatomic ROIs—due to differences in spatial scan parameters and resolution (eg, section thickness, gap)—further complicates analysis, both in terms of in-plane resolution and along the z-axis (with associated partial volume averaging effects).

Finally, correlation of the observed MRS metabolite ratio changes with either PET or follow-up MR imaging scans would be paramount to affirm our preliminary findings. While decreased tCho/NAA ratios posttreatment suggest an antiproliferative effect, this inference cannot be determined unequivocally. However, these 18 patients are part of an ongoing phase II study at our institution that includes a larger patient sample size, PET scans, and continued interval MR imaging follow-up, designed to assess PFS and overall survival rates in patients receiving IA bevacizumab therapy. Histologic confirmation would have been a powerful adjunct in validating our findings; however, tissue sampling was not performed, given the observed overall favorable radiographic treatment responses.