99mTc-labeled nucleotides as tumor-seeking radiodiagnostic agents.

Abstract

Several lines of human tumor cells in monolayer and soft agar cultures allow permeation of low levels of adenine nucleotides through their plasma membranes, while, in general, untransformed cells do not incorporate adenine nucleotides into their cellular pools without prior degradation of the nucleotides to adenosine. This study determined the uptake of "99Tc-radiolabeled chelated forms of adenine nucleotides, "'Tc-Ap4A (diadenosine 5',5"',P',P4-tetraphosphate) and ""'Tc-ATP chelates as radiodiagnostic agents suitable for the in vivo detection of tumors by radionuclide imaging. Biodistribution studies revealed that Ap4A accumulated preferentially in RT-24 tumors implanted in rats and that V2 carcinoma implanted in rabbits could be readily visualized by in vivo imaging. The biodistribution at various time points showed increased tumor-to-muscle ratios after ""'Tc-ApsA or 9Tc-ATP injections when compared with a nonspecific marker of the extracellular fluid space, 99mTc-labeled diethylenetriaminepentaacetic acid and with an agent known to localize in some tumors, 67Ga-labeled citrate. Normal mammalian cells are usually impermeable to nucleotides in the surrounding medium (3, 4). Nevertheless, mammalian cells can be infected with low efficiency by viral nucleic acids (5), and mouse fibroblasts (L cells) were shown to be permeable to intact dAMP and araAMP (6), as well as other deoxynucleoside monophosphates (7). Incorporation of low concentrations of nucleotides into cellular pools is impaired by the activity of ectoenzymes such as ATPase, ADPase, alkaline phosphatase, 5'-nucleotidase, and nucleotide pyrophosphatase (8, 9). These enzymes are integral plasma membrane proteins with an active site situated on the outside of the cell (10). It has been reported that the activity of these enzymes fluctuates with the degree of cellular proliferation and differentiation (8).The permeability of tumor cells to intact adenine nucleotides could provide a diagnostic marker useful for the in vivo localization of tumors if an analog could be labeled with a useful gamma-emitting radionuclide. 99mTc is widely used for radionuclide imatng in humans because of its desirable physical properties: mTc is metastable, has a 6-hr physical half-life, and has an 85% incidence of a detectable gamma photon at 140 keV. These physical properties, coupled with scintillation cameras that are optimized for the 140 keV energy of 99mTc, make`mTc the preferred radionuclide for scintillation imaging in humans (11) Cells. 3T3 (BALB/c mouse fibroblast), SV-3T3 (simian virus 40-transformed 3T3 cells), BHK 21/13 (baby hamster kidney 21/13), Py-BHK 21/13 (polyoma virus-transformed BHK 21/13), SV-BHK, and C3A (human hepatoma) cells were cultured in Dulbecco's modified Eagle's (DME) medium supplemented with 10% fetal bovine serum. Chicken embryo fibroblasts were cultured in DME medium supplemented with 5% fetal bovine serum/10% tryptose phosphate broth. Preparation of primary chicken embryo fibroblasts and their infection with Rous sarcoma virus were c...

Analogues of diadenosine 5',5'''-P1,P4-tetraphosphate (Ap4A) as potential anti-platelet-aggregation agents.

“…More recently it has been recognized that Ap4A and its analogues play an important role in extracellular signaling events as well as in specific enzyme inhibition (7,(28)(29)(30)(31)(32)(33)(34)(35)(36)(37). These extracellular functions draw attention to the possible role of Ap4A and its analogues as pharmacologically active compounds in cardiovascular, neurotransmitter, stress-related, and diverse purinergic fields.…”

Detection of inflamed atherosclerotic lesions with diadenosine-5′,5‴- P 1 , P 4 -tetraphosphate (Ap 4 A) and positron-emission tomography

“…In this study, we evaluate the imaging capabilities of positron-emission tomography (PET) with P 2 ,P 3 -[ 18 O ur previous findings with 99m Tc-conjugated diadenosine-5Ј,5ٞ-P 1 ,P 4 -tetraphosphate (Ap 4 A) and P 2 ,P 3 -monochloromethylene diadenosine-5Ј,5ٞ-P 1 ,P 4 -tetraphosphate (AppCHClppA) in a rabbit model indicated rapid, high accumulation of both diadenosine tetraphosphate analogs in the atherosclerotic abdominal aorta and demonstrated up-regulation of purine receptors in experimental atherosclerotic lesions, suggesting the potential for using such labeled analogs for noninvasive detection of plaque formation (1,2). The atherogenic process involves sequestration of partially oxidized lipids in the vessel wall (3,4), leading to endothelial injury that promotes adherence of mononuclear cells and platelets and contributes to phenotypic transformation of medial smooth muscle cells (SMCs) from adult to embryonic forms.…”

Rapid noninvasive detection of experimental atherosclerotic lesions with novel 99m Tc-labeled diadenosine tetraphosphates

“…The method for radiolabeling Ap 4 A described here resulted in a purer and more consistent product than a previously reported procedure (18). This procedure described above that used glucoheptonate as the coligand yielded a mixture of the 99m Tc-glucoheptonate (retention time 2-3 min, see above) and 99m Tc-Ap 4 A-glucoheptonate at 16 min.…”