Applications of magnetic resonance in model systems: cancer therapeutics

Abstract

The lack of information regarding the metabolism and pathophysiology of individual tumors limits, in part, both the development of new anti-cancer therapies and the optimal implementation of currently available treatments. Magnetic resonance [MR, including magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), and electron paramagnetic resonance (EPR)] provides a powerful tool to assess many aspects of tumor metabolism and pathophysiology. Moreover, since this information can be obtained nondestructively, pre-clinical results from cellular or animal models are often easily translated into the clinic. This review presents selected examples of how MR has been used to identify metabolic changes associated with apoptosis, detect therapeutic response prior to a change in tumor volume, optimize the combination of metabolic inhibitors with chemotherapy and/or radiation, characterize and exploit the influence of tumor pH on the effectiveness of chemotherapy, characterize tumor reoxygenation and the effects of modifiers of tumor oxygenation in individual tumors, image transgene expression and assess the efficacy of gene therapy. These examples provide an overview of several of the areas in which cellular and animal model studies using MR have contributed to our understanding of the effects of treatment on tumor metabolism and pathophysiology and the importance of tumor metabolism and pathophysiology as determinants of therapeutic response.

Figure 1

¹H-decoupled ³¹P MR spectra of extracts from (A) control L1210 cells and (B) apoptosing L1210 cells following 3 hours of treatment with 50 µM nitrogen mustard. Spectra were acquired on a 250-MHz Bruker spectrometer and are the result of 10,000 fully relaxed scans plotted with a line broadening of 0.5 Hz. F-1, 6-P: fructose-1, 6-bisphosphate; G-3-P: glycerol-3-phosphate; DHAP: dihydroxyacetone phosphate.

Figure 2

Relationship between the “dose”, as indicated by the therapeutic effect (or STGD), and the “response”, as indicated by the maximum increase in tumor water ADC for two chemotherapeutic agents (cyclophosphamide and 5-FU) and three tumor lines (RIF-1, Colon-26 and Colon-38).

Figure 3

(A) ³¹P MR spectra of perfused RIF-1 tumor cells obtained during the first 1 to 5, 6 to 10 and 11 to 15 hours of perfusion with 40 µM 6-AN. Peaks are identified as follows: 1=6PG, 2=phosphoethanolamine, 3=PCho, 4=P_i 5=phosphodiesters, 6=PCr, 7, 8, and 9 are γ-, α-, and β-NTP. Over the course of the infusion, PCr and β-NTP decrease and 6PG, which is not present initially, increases to become the dominant peak in the spectrum. Note that the P_i peak is primarily from the perfusate. (B) ¹³C MR spectra of RIF-1 cells immediately (bottom) and 2 hours after (top) perfusion with 40 µM 6-AN and 1-¹³C glucose. Two peaks, assigned to 6PG and 6-phosphoglucono-D-lactone, are detected both during and after perfusion and in perchloric acid extracts (not shown).

Figure 4

³¹P MR spectra of MCF7 tumors grown as xenografts in the mammary fat pads of female SCID mice. 3-APP: 3-aminopropylphosphonate; PME: phosphomonoesters; P_i: inorganic phosphate; NTP: nucleoside triphosphates. Representative spectra from a control mouse given normal drinking water and a mouse provided ad libitum water containing 200 mM NaHCO₃ are shown.

Figure 5

Effects of radiation on pO₂ and volume of tumors. Twenty Gy of ionizing radiation was given as a single dose or as a split dose with the interval between the two 10 Gy doses determined by the time course of changes in mean tumor pO₂. Treatment groups were: ○-Control (Sham-Irradiated); □-Single (20 Gy at time 0); ●-Oxygenated (10 Gy at time 0 and 10 Gy at 72 hours); ▲-Hypoxic (10 Gy at time 0 and 10 Gy at 24 hours). Points are the mean of five mice per group; error bars are SEM of measurements from five tumors at each time point. Left side: Changes in pO₂ as measured by EPR oximetry. Right side: Tumor volume as determined by physical measurements of three axes (revised from Ref. [85]).

Figure 6

Correlation of MR and oxygen microelectrode data: Water signal peak height in a rodent mammary adenocarcinoma under control conditions. The subsequent images show the difference between control images and the change in water signal peak height during carbogen breathing. The changes are spatially inhomogeneous and occur primarily in the tumor. The graphs below show changes in pO₂ measured by oxygen microelectrode during carbogen breathing in the same tumor. The region in which MR signal peak height does not change also shows no change in microelectrode current

Figure 7

Representative in vivo ¹⁹F spectra from subcutaneous tumors obtained 120–150 minutes following i.p. injection of 1 g kg^-1 5-FC. A single peak of 5-FC is observed in the parental HT29 carcinoma (A). In contrast, the HT29 carcinoma expressing yCD (B) converted the 5-FC to 5-FU, which was subsequently metabolized into fluorinated nucleotides (Fnuc). Fluoro-β-alanine (FβAI), a catabolic breakdown product of 5-FU, was also observed in this tumor. Reprinted from Stegman L.D. et al. [98]. Non-invasive detection of cytosine deaminase transgene expression in human tumor xenografts with in vivo magnetic resonance spectroscopy [PNAS 96: 9821–9826. National Academy of Sciences, U.S.A. Copyright (1999).]