Whether Ga-67 accumulates in tumors by transferrin (Tf-dependent or independent methods has been a topic of controversy for years. Our preliminary data suggests that accumulation of Ga by most normal tissues and organs requires the presence of Tf and the transferric receptor (TfR), but that Ga uptake by tumors may be, in many cases, a Tf-independent process. Further, we have demonstrated that the Tf-independent uptake of gallium by tumor cells in-vitro can be increased significantly by a number of manipulations, the most powerful of which is incubation of the cells with nifedipine, a calcium channel blocker, that has been pre-exposed to either fluorescent or UV light. In this grant application, we will explore the mechanism and utility of photo-degraded nifedipine (PDN) in mediating Ga-67 uptake. We will determine which of the photodegradation products of nifedipine are responsible for the action on Ga-67 uptake. We will isolate the products of UV and fluorescent photo-irradiation of nifedpine chromatographically. We will characterize their physical properties by HPLC, NMR, gas chromatography-mass spectroscopy, and UV and fluorescence spectroscopy. We will assess the activity of the most abundant isolates on mediating uptake of Ga-67 in cells. For those derivatives that prove most abundant and active in promoting uptake, we will compare and define their stabilty and continued activity in promoting Ga-67 uptake under conditions of extended exposure to light and to a variety of conditions encountered in storage. With the most active, abundant and stable derivatives, we propose to define the mechanism by which PDN promotes Ga-67 uptake. Based on other reports of PDN activities, and our own observations, we will focus on the 4 most-likely ways that PDN may mediate Ga-67 uptake: (1) by means of a calcium channel; (2) by activating of a Ca2+-mediated calcium channel; (3) by an effect of nitric oxide production; or (4) by an effect related to a receptor-mediated function. We will also determine whether PDN has other biological or physiological properties that might expand their utility or complicate their clinical use. The toxicity of PDN will be assessed. The effect of these agents on the distribution of Ga-67 in normal tissues in mice will be determined, and whether the agents allow earlier imaging with less activity. The effect of nifedipine and PDN on the uptake of Ga-67 by a variety of types of tumors grown as explants of cultured cells in mice will be evaluated. This will enable us to assess whether PDN mediates uptake of Ga-67 in all tissues, or only in cancer. By comparing the magnitude of uptake in tumors of varying types, we can assess how broadly applicable PDN may be for enhancing uptake of Ga-67 in tumors. Even if the stimulatory effect on Ga-67 uptake proves not specific to tumors, but is found in all tissues, PDN may nonetheless prove useful in shortening the time required from injection of Ga-67 to imaging, and may allow diagnostic images to be obtained at a lower dose of the radionuclide.
