Targeting of ultrasmall superparamagnetic iron oxide (USPIO) particles to tumor cells in vivo by using transferrin receptor pathways.

Human transferrin was covalently coupled to ultrasmall superparamagnetic iron oxide (USPIO) particles, and the transferrin-USPIO obtained was investigated in vivo in experimental SMT/2A tumor-bearing rats (rat mammary carcinoma). Physicochemical characterization showed an overall size of 36 nm (DLS) with a core size of 5 nm (TEM). Relaxivities were R1 = 23.6 and R2 = 52.1 liter/mmol.s (0.47 T). Bound transferrin was 280 micrograms/mg of iron. Pharmacokinetic investigations revealed a half-life of 17 min in normal rats. The MR evaluation of tumor signal intensity over time showed a 40% (range 25-55%) signal reduction 150 min after injection with the reduction persisting for at least 8 h. Control experiments using the parent USPIO compound or USPIO labeled with a nonspecific human serum albumin (HSA-USPIO) showed a change of only 10% (range 5-15%) in tumor signal intensity over time. The results demonstrate that a combination of the USPIO relaxivity properties with the specificity of transferrin-mediated endocytosis allows in vivo detection of tumors by MR imaging.

Magnetic resonance lymphography in rats: effects of muscular activity and hyperthermia on the lymph node uptake of intravenously injected superparamagnetic iron oxide particles.

RATIONALE AND OBJECTIVES: We investigated the influence of muscular activity and regional body temperature changes on the accumulation of intravenously (i.v.) administered, dextran-coated superparamagnetic iron oxide (SPIO) particles in the lymph nodes of rats. METHODS: Four groups of rats (N = 21) were used. Five rats were allowed to move freely after i.v. contrast administration (group 1). In another five rats, muscular inactivity (group 2) was induced during i.v. injection of SPIO particles and for up to 2 hr thereafter by anesthesia. In seven rats (likewise anesthetized), the contrast agent was administered while the extremities of one side of the body were warmed in a water bath for 2 hr (group 3). The rats in groups 1-3 received 100 mumol Fe/kg of the contrast agent. Four rats not given SPIO particles served as the control group (group 4). The lymph nodes of all animals were removed 24 hr after SPIO administration and were embedded in an agar matrix for magnetic resonance imaging at 1.5 T using a proton-density-weighted spin-echo (PD-SE) sequence and a T2*-weighted gradient-recalled echo (T2* GRE) sequence. RESULTS: Signal loss varied widely among the different lymph nodes in group 1. A pronounced signal reduction was observed in the mesenteric (PD-SE = 20 +/- 6%, T2* GRE = 55 +/- 19%), iliac (PD-SE = 13 +/- 13%, T2* GRE = 44 +/- 24%), and popliteal (PD-SE = 24 +/- 7%, T2* GRE = 70 +/- 11%) lymph nodes and only a moderate reduction in the mandibular (PD-SE = 4 +/- 7%, T2* GRE = 42 +/- 15%), axillary (PD-SE = 0 +/- 4%, T2* GRE = 8 +/- 7%), and inguinal (PD-SE = 5 +/- 5%, T2* GRE = 34 +/- 18%) lymph nodes. The least pronounced signal loss occurred in the peripheral lymph nodes of group 2, ranging from 0 +/- 3% for PD-SE sequences and 10 +/- 11% for T2* GRE sequences to 13 +/- 15% for PD-SE sequences and 41 +/- 19% for T2* GRE sequences. In group 3, the uptake of contrast material in the peripheral lymph nodes of the hyperthermal side was significantly more pronounced than on the contralateral side (p < .01), and the contrast agent was distributed more evenly to the different lymph node groups than in group 1. CONCLUSION: Muscular activity and regional hyperthermia markedly influence the accumulation of SPIO particles in different lymph node groups in rats. These findings must be considered in preclinical studies and in the clinical administration of MR lymphography.

Paramagnetic metalloporphyrins: infarct avid contrast agents for diagnosis of acute myocardial infarction by MRI.

In previous experiments in tumors we demonstrated that metalloporphyrins are particularly avid for nonviable tumor components. This study was performed to find out whether these agents can be used as MRI contrast agents for the visualization of acute myocardial infarction (MI). A total of 44 rats, 6 normal controls and 38 with occlusive MI (2-24 h old), were used. Gadolinium mesoporphyrin (Gd-MP) or manganese tetraphenylporphyrin (Mn-TPP) was intravenously injected at doses of 0.1, 0.05, and 0.01 mmol/kg. Three to 24 h after injection, axial and coronal T1-weighted (TR/TE 300/15 ms) spin-echo MR images were obtained before and after killing the animals and correlated with triphenyl tetrazolium chloride (TTC) histochemical preparations. The Gd-MP content in infarcted and noninfarcted myocardium was measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). MRI without contrast media could not discern the MI. However, 3-24 h after injection of either Gd-MP or Mn-TPP, the infarcted area was positively stained on MR images. This area matched well with the negatively TTC-stained area on the heart slices (r = 0.97). The contrast ratios between the infarcted necrotic myocardium and the noninfarcted regions varied from 150 to 300% depending on the type of agents and doses used. Neither false-positive nor false-negative findings were encountered. The metalloporphyrin concentration was more than 10 times higher in the infarcted than in the noninfarcted heart. Metalloporphyrins appear to be promising MRI contrast agents for detection and quantification of necrosis in MI. These preclinical results may open new perspectives in cardiac imaging.

Interstitial MR lymphography with iron oxide particles: results in tumor-free and VX2 tumor-bearing rabbits.

OBJECTIVE: MR lymphography with interstitial injection of superparamagnetic iron oxide particles was optimized in normal rabbits by investigating the pattern of signal reduction in lymph nodes as a function of dose and time after administration. The optimized examination procedure was then used in a rabbit tumor model to study the potential of superparamagnetic iron oxide--enhanced MR lymphography in the detection of metastatic lymph node involvement. MATERIALS AND METHODS: The popliteal and iliac lymph nodes of 18 normal rabbits were imaged to study the dose response and time course of the effect of the contrast agent. For the dose response study, six doses of 2-50 mumol of iron per extremity were administered to three animals per dose, and MR images were obtained before and 12 hr after administration. For the time course study, 20 mumol of iron per extremity was administered to four animals, and images were obtained before and 6 hr to 42 days after administration. VX2 tumor-bearing rabbits were examined 12 hr after administration of 20 mumol (10 animals) and 50 mumol (three animals) of iron per extremity. Superparamagnetic iron oxide was injected into the foot pad of the hind limb. T1-, T2-, and proton density-weighted MR images were obtained with a 1.5-T unit. RESULTS: In normal rabbits, a profound and homogeneous loss of signal intensity was found with doses of 2-5 mumol of iron per extremity in popliteal lymph nodes and with doses of 20-30 mumol in the iliac lymph nodes. Superparamagnetic iron oxide caused maximal loss of signal intensity in both popliteal and iliac lymph nodes 12 hr after administration. In tumor-bearing rabbits, different degrees of metastatic displacement of lymph nodes were discernible, and even small metastases (3 mm in diameter) could be visualized when using the optimized examination protocol and the proton density-weighted spin-echo sequence. CONCLUSION: We conclude that interstitial MR lymphography with superparamagnetic iron oxide enables the detection of lymph node metastases and therefore is a promising technique for improved diagnostic imaging of lymph nodes in the staging of tumors.