Direct imaging of the uptake of platinum anticancer agents using X-ray stimulated fluorescence: A proof-of-concept study

Impact/Innovation Platinum (Pt)-based drug is one of the most widely used and effective anticancer agents. It plays a major role in the treatment of a variety of cancers. Despite an expanding panel of Pt drugs, tumor cell resistance to chemotherapy agents continues to pose a significant challenge in the management of these neoplasms. Previous studies have suggested several mechanisms of resistance, the most common phenotype of which is decreased Pt drug accumulation in the tumor region due to either decreased influx or increased efflux 1-3. Thus, a successful Pt-based cancer chemotherapy is highly dependent on the ability to assess drug uptake and retention in vivo, which is still infeasible with current imaging modalities. According to the Response Evaluation Criteria in Solid Tumors (RECIST), response to treatment during Pt-based chemotherapy is usually assessed by changes in tumor volumes measured with conventional anatomical imaging techniques (MRI or CT). However, changes in tumor volume may not reliably identify patients with a favorable histological response to therapy. Some solid tumors do not change size regardless of the success or failure of treatment. In addition, necrotic cells may swell and actually enlarge the tumor volume, or these cells may not be eliminated rapidly from the tumor itself. These aspects of cancer biology emphasize limitations of the current model for determining effects of Pt based chemotherapy through changes in tumor volumes and have compelled oncologists to call for new, imaging-based methods to answer this critical clinical question. On the other hand, radiological RECIST measurements typically support accurate detection of response only after 8-10 weeks of chemotherapy, i.e. after the first part of chemotherapy is finished. Since Pt-based chemotherapy is ineffective for some patients, these patients will endure toxic effects of treatment with no therapeutic benefit. Moreover, for patients who do not respond to chemotherapy, the window of opportunity has passed for switching to an alternate protocol that may produce a more favorable response, and increase the likelihood for cure.

More recently, 18F-FDG PET/CT imaging has allowed for significant improvements in treatment monitoring, since FDG images tumor glucose metabolic rate. Significant decreases in FDG uptake and retention at tumor sites often correlate with the long-term patient outcomes. However, none of PET tracers are directed at changes in the uptake and intracellular retention of Pt drugs, which is one of the direct indicators for Pt treatment response. Especially, during the course of concurrent Pt-based chemotherapy and radiotherapy, the imaging surrogate endpoint for therapy responder or partial responder, i.e. a reduction in FDG uptake and retention, cannot be used to effectively assess the treatment efficacy attributable to the Pt-based chemotherapy alone. Thus, developing an early detection strategy for the delivery of Pt drugs in vivo is crucial to contain the disease, which would offer the potential to tailor treatment to individual patients nearly in real time, thereby substantially improving patient survival and quality of life. In this study, the introduction of energy-dispersive X-ray-stimulated fluorescence CT (XSF-CT) to directly image the uptake and retention of Pt drugs in the tumor regions receiving chemotherapy would allow us to track the pharmacokinetics of Pt drugs throughout the treatment process. Pt is an element with high atomic number (Z).

Bombarding the tumor regions where Pt-based drugs accumulate with X-rays will excite the emission of characteristic Pt X-rays (i.e. XSF of Pt) through the photoelectric process (Fig. 1). This technique gives the total amount of Pt drugs regardless of the chemical forms or derivative metabolites present in the tumor. A similar scenario occurs when we image the uptake and retention of FDG in the tumor region using PET. Knowledge of the delivery of Pt drugs in the tumor region can also provide important clinical information when optimizing treatment to obtain maximum therapeutic effect, minimal toxicity, and reduced cost, and to enable switching from ineffective therapies for personalized medicine. Supporting data As a proof-of-concept, the XSF-CT acquisition was performed in a first generation CT geometry, acquiring a single line integral at a time (Fig 2). A 5mm X-ray pencil beams was used to selectively excite a slice of the water phantom. The water phantom embedded with 2% (w/v) Pt, gadolinium (Gd) and iodine (I) solutions was translated 30 times in increments of 1.5 mm, and rotated 31 times to cover 360°. We included Gd and I in the water phantom to demonstrate the capability of XSF-CT to resolve the XSF signals

Fig. 1. XSF generation. A K-shell electron is removed from an element by a beam of photons through the photoelectric process. The vacancy in the K shell is subsequently filled by an electron from the L or M shells. The energy difference between these shells is released as either an Auger electron or a fluorescent X-rays characteristic of the element. In elements with an atomic number Z ≥ 47, transitions from L and M to K are accompanied by characteristic K X-rays ≥ 80% of the time.

from three elements with a single scan. Dynamic contrast-enhanced MRI using Gd as the contrast agent has been used to assess the tumor capillary permeability for the delivery of Pt drugs during concurrent Pt-based chemotherapy and radiotherapy. Radioactive I-124 PET imaging has also been used to track the

targeted therapy of

cancer. Integrating the

multiplexing imaging of Pt, Gd and I into a single platform of XSF-CT offers the

potential to simultaneously image multiple phenotypes of chemo/radio therapy with undeniable advantage since multiplexing imaging is simultaneous, not sequential (creating artifacts caused by intra- and inter-scan displacement, patient and organ motion), which may affect the method accuracy in certain circumstances. XSF photons emitted from the narrow strip of volume illuminated by the pencil beam were collected using a cadmium telluride (CdTe) detector. The detector system was placed at 90 to the incident X-ray beam direction to minimize the unwanted scattered photons entering the detector. The K shell XSF photons were detected and form a spectrum (photon energy distribution) for one projection. The data acquisition of XSF-CT resulted in a set of spectra. The spectra were used for the K shell XSF peak isolation and sinogram generation, for Pt, Gd and I, respectively. The individual image for the three elements was reconstructed from the corresponding sinogram separately (Fig. 3 and 4).

A fairly good linear relationship between XSF intensity and the concentrations of Pt, Gd and I solutions were also observed in Fig 5(a)(b)(c), respectively, suggesting that XSF-CT is capable of quantitative imaging. The X-ray dose from the XSF-CT imaging procedure was also measured with TLDs. The X-ray dose at the center of the water phantom was 0.25 cGy per projection. Currently,

new strategies for Pt XSF imaging are under investigation. References 1. R. P. Perez, "Cellular and molecular determinants of cisplatin resistance," Eur J Cancer 34, 1535-1542 (1998). 2. J. Chen, N. Emara, C. Solomides, H. Parekh and H. Simpkins, "Resistance to platinum-based chemotherapy in lung cancer cell lines," Cancer Chemother Pharmacol 66, 1103-1111 (2010). 3. P. J. Ferguson, "Mechanisms of resistance of human

tumours to anticancer drugs of the platinum family: a review," J Otolaryngol 24, 242-252 (1995).

Fig. 2. Schematic of the experimental setup including the filtered X-ray pencil beam, the water phantom and the CdTe detector. Inset: multiplexing spectrum of Pt, Gd, Barium and I. The area under the blue dot line was the background photons derived from the Compton scatter.

Figure 3. Reconstructed XSF multiplexed images of 2% (w/v) Py, Gd and I solution in a water phantom. (a) Pt; (b) Gd; (c) I; (d) multicolor overlay. The water phantom (d, left) was scanned by the XSF-CT. The XSF spectrum in a projection was obtained by detecting the XSF photon emitted by all pixels along the beam (left column at (a)(b)(c)). The XSF peaks in the spectra were processed into a sinogram for each element (middle column at (a)(b)(c)), which was reconstructed with 100 iterations of ML-EM (right column at (a)(b)(c)). Pseudo color in (d) for unmixing components: Red-I; Blue-Pt; Green-Gd.

Fig 4. Co-registration of the XSF-CT emission image and X-ray transmission CT images. The X-ray transmission images of the water phantom scanned with an on board imager of the Varian Truebeam Linear Accelerator.

Fig 5. A linear relationship was found between the XSF intensity and the concentration of the element solution: (a) Pt; (b) Gd; (c) I.