Abstract CN01-02: Incorporating molecular imaging methods in early drug development
Authors: Vries E. G. Elisabeth De , Brouwers Adrienne , Hooge Marjolijn N. Lub-de , Schroder Carolien P. , Hospers Geke AP , Gietema Jourik A. 
Published: May 2016

In drug development and clinical trials attention is paid not only to pharmacokinetic analysis of the drug, but also to tumor characteristics in tumor biopsies in order to obtain optimal patient selection. However this approach lacks information concerning three important issues, namely heterogeneity of tumor characteristics, whether the drug reaches the tumor lesions and whether the optimal dose of the drug is administered. Molecular imaging with radionuclide imaging and positron emission tomography (PET) or with a fluorescent probe and optical imaging can potentially provide interesting and relevant information for these issues.

Heterogeneity of relevant tumor characteristics is increasingly understood to be of relevance. Most hallmarks of cancer can now be imaged by specific tracers, with single-photon emission computed tomography (SPECT) and PET. PET-scanning offers improved resolution and easier quantification. Drugs can also be labeled in order to visualize their targets, for example 89Zr-trastuzumab in case of HER2. Several studies with tracers such as 89Zr-trastuzumab and 18F-fluoroestradiol (FES) show presence of significant heterogeneity in uptake of tracers in tumor lesions within a patient. Visualization of the estrogen receptor (ER) with FES is feasible in both breast and ovarian cancer patients. Heterogeneity of ER expression occurs in breast cancer as both FES-positive and -negative lesions are present in 15-47% of patients with an earlier ER-positive primary tumor.

Knowledge about presence of less well-known drug targets can also be of interest. For example several mesothelin-targeting agents including antibody-drug conjugates are evaluated in trials. Recently the first SPECT and PET studies with radiolabeled mesothelin antibodies have been performed. This specific information can be used to guide further early drug development.

Most targeted agents are specifically designed to accumulate in the tumor, which makes it even more important to have information about the time course of drug presence in tumor lesions. Even when a specific drug target is present in a tumor, the drug may still fail to reach this target, for a multitude of reasons. SPECT imaging detected tumor uptake of the labeled DR5 directed antibody 111In-CS-1008 in only 63% of the patients, despite DR5 expression in all archival tumor tissues analyzed (Ciprotti M, J Clin Oncol 2015). Similar results were seen in HER2-positive metastatic breast cancer patients in which no or minimal tracer uptake of 89Zr-trastuzumab on PET/CT scan was seen in most of the metastatic tumor load in 32% of patients. These findings may be due to absence of the drug target across all lesions or because the lesions are not reached by the drug. These PET findings may mean that these patients will not benefit from monoclonal antibody based treatment. Of course, when the drug does reach the target an antitumor effect can still be absent. In this respect early FDG-PET scanning can be a functional effect sensor.

Serial imaging for drug effects and target saturation can be performed for antibodies. We evaluated the feasibility of using 89Zr-trastuzumab-PET to determine in vivo degradation of HER2 caused by the novel HSP90 inhibitor NVP-AUY922. HSP90 chaperones have key client proteins that are involved in all hallmarks of breast cancer growth and progression. The standard uptake value (SUV) max change in individual tumor lesions from the baseline to the 3 weeks 89Zr-trastuzumab-PET was heterogeneous and related to size change on CT after 8 weeks of NVP-AUY922 treatment. It was concluded that NVP-AUY922 therefore showed proof-of-concept clinical response in HER2-amplified metastatic breast cancer. But also other tracers can be used to prove that the drug target is no longer available. For example, tumor estrogen uptake can be measured with FES and PET scanning. During fulvestrant therapy, serial imaging of tumor estrogen uptake provided insight into the effect of standard dosing with the ER antagonist fulvestrant on abolishment of estradiol uptake. FES-PET showed residual ER availability in tumor lesions in 38% of the patients, which was associated with early disease progression. The androgen receptor (AR) has been imaged with 18F-fluorodihydrotestosterone (FDHT) in prostate cancer patients. FDHT-PET showed that the AR antagonist enzalutamide substantially reduces FDHT binding (Scher H, et al. Lancet. 2010). The AR is increasingly considered of interest in breast cancer and a trial with FDHT-PET in breast cancer is ongoing

Especially during drug development, when repeat biopsies are performed, it might be interesting and informative to use fluorescent-labeled drug tracers. The uptake can be visualized with a number of imaging platforms, and the precise location within the tumor lesions can be verified with fluorescence microscopy. Combining fluorescent and PET tracers may be very interesting for immunotherapy trials, as tumor cell characteristics and presence and activation of immune cells in the tumor do not remain static over time. It is expected that molecular imaging may be able to provide major insights at the tumor level before and during novel immunotherapies. This may be exploited to optimally sequence different immunotherapies.

More general implementation of molecular imaging comprises challenges for the tracer development and manufacturing, regulatory aspects, as well as technological issues and costs. To implement molecular imaging, standardized PET scanning is needed. Sharing knowledge, for example by publishing tracer development (including manufacturing and quality control) procedures, will expedite this process. International multidisciplinary collaborations between academia and industry are crucial. Several initiatives are underway to achieve widespread standardization and harmonization of scanners, scanning- and evaluation procedures. This strategy can also contribute to the creation of a dedicated warehouses containing PET scans with different tracers. This will also enable novel radiomics and radiogenomics approaches.

Thus, molecular imaging can identify characteristics of a specific tumor across the entire body over time, can provide new mechanistic and pharmacological insights advantageous for drug development and can contribute to identify patients most likely to benefit from a specific treatment.

Citation Format: E. G. Elisabeth De Vries, Adrienne Brouwers, Marjolijn N. Lub-de Hooge, Carolien P. Schroder, Geke AP Hospers, Jourik A. Gietema. Incorporating molecular imaging methods in early drug development. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr CN01-02.