Comparison of dendrimer-based macromolecular contrast agents for dynamic micro-magnetic resonance lymphangiography.

Few methods are currently available to visualize the entire lymphatic system. A method known as micro-magnetic resonance lymphangiography (MRL), which employs a dendrimer-based MRI contrast agent (PAMAM-G8) and a clinical-grade 1.5T MRI instrument, was recently developed for use in mice. In the present study, three dendrimer-based MRI contrast agents (PAMAM-G8, DAB-G5, and PAMAM-G4) with different pharmacokinetic characteristics were compared to determine the best reagent to visualize the lymphatic system under physiological or pathological conditions. In addition, two established MRI contrast agents (Gadomer-17 and Gd-[DTPA]-dimeglumine (Magnevist)) were used as control agents. In experiments with mice, most of the deep lymphatic system was visualized by micro-MRL with all agents except Gd-[DTPA]-dimeglumine. PAMAM-G8 was best for visualizing lymphatic vessels, whereas DAB-G5 was better for visualizing lymph nodes. PAMAM-G4 was intermediate in character between PAMAM-G8 and DAB-G5, except in exhibiting a low background signal (especially in the liver). The lymphatic system was not clearly visualized with Gd-[DTPA]-dimeglumine; however, the lymph nodes were visualized with Gadomer-17, although not as well as with dendrimer-based agents. In conclusion, DAB-G5 and PAMAM-G4 can be used to identify lymph nodes and lymphatic vessels, respectively. Their rapid excretion makes these compounds potentially attractive for human use.     

Characterization of N-ethyl-N-nitrosourea-induced malignant and benign breast tumors in rats by using three MR contrast agents

A carcinogen (N-ethyl-N-nitrosourea)-induced animal tumor model was established to grow malignant and benign breast tumors. In each tumor the pharmacokinetic characteristics were measured by using three contrast agents, gadolinium-diethylene-triamine-pentaacetic acid (Gd-DTPA; <1 kD), Gadomer-17 (35 kD), and albumin-Gd-DTPA (70-90 kD). Infiltrating ductal carcinomas (IDC) with low, medium, and high Scarf-Bloom-Richardson grades and fibroadenomas (FA) were analyzed. We found that Gd-DTPA could differentiate between FA and malignant tumors, but not between malignant tumors of low and high grades. In contrast, the intermediate size agent Gadomer-17 could differentiate between high-grade and low-grade IDC, but not between low-grade IDC and FA due to their similar enhancement patterns (despite their different origins). The largest agent, albumin-Gd-DTPA, was capable of differentiating both, but the low contrast-to-noise ratio was its major technical concern. The results in this breast tumor model suggest that macromolecular agents provide useful information for differential diagnosis among IDCs of various grades, but they do not provide superior information than Gd-DTPA for differential diagnosis between IDC and FA.     

一种磁共振造影剂前体BSA-(Gd-DTPA)_n的制备、表征及体内外评价

目的:改进BSA-(Gd-DTPA)_n制备及纯化方法,探讨其作为磁共振造影剂前体的优势及应用前景.方法:BSA与二乙烯三胺五乙酸环酐反应生成BSA-(DTPA)_n,并与GdCb螯合生成BSA-(Gd-DTPA)_n.紫外光谱法鉴定其结构,并定量测定其中DTPA对BSA的偶联率.测定配合物体外弛豫时间T_1、T_2,分析其弛豫性能R1、R2.小鼠急性毒性试验评价药物安全性.大鼠磁共振增强扫描评价其活体内代谢及分布情况.结果:本研究制得的BSA-(Gd-DTPA)n配合物中n=26.体外弛豫性能RI约为7.00×10~(-3) L·mmol~(-1)·ms~(-1).比小分子Gd-DTPA的弛豫性能(3.52×10~(-3) L·mmol~(-1)·ms~(-1))提高近1倍.大鼠磁共振增强扫描显示BSA-(Gd-DTPA)-n和白蛋白一样具有长循环特性.结论:本实验改进方法可制备出磁共振造影剂前体BSA-(Gd-DTPA)_n,该配合物具备长循环特性,且具有多个可供修饰的氨基,可作为一种潜在的磁共振造影剂前体.     

Comparison of single- and dual-tracer pharmacokinetic modeling of dynamic contrast-enhanced MRI data using low, medium, and high molecular weight contrast agents.

Pharmacokinetic parameters corresponding to perfused microvascular volume determined from dynamic contrast-enhanced (DCE) MRI data were compared to immunohistochemical measures of microvascular density (MVD) and perfused microvascular density. DCE MRI data from human mammary tumors (MDA-MB-435) implanted in nude mice using low (Gd-DTPA, MW approximately equal 0.6 kDa), medium (Gadomer-17, MW(eff) approximately equal 35 kDa), and high (PG-Gd-DTPA, MW approximately equal 220 kDa) molecular weight contrast agents were analyzed with single- and dual-tracer pharmacokinetic models. MVD values were determined by two manual counting methods, "hot spot" and summed region of interest (SROI). Pharmacokinetic parameters determined using the single-tracer model (Gd-DTPA [n = 15] and Gadomer-17 [n = 13]) did not correlate with MVD measures using either manual counting method. For dual-tracer studies (Gadomer-17/Gd-DTPA [n = 15] and PG-Gd-DTPA/Gd-DTPA [n = 13]), pharmacokinetic parameters demonstrated a statistically significant correlation with MVD determined by the SROI method, but not the "hot spot" method. Ten mice successfully underwent intravital FITC-labeled lectin perfusion with the hemisphere of highest lectin labeling correlating with pharmacokinetic parameter values in 9 of 10 tumors (single-tracer Gd-DTPA [n = 2], single-tracer Gadomer-17 [n = 3], and dual-tracer Gadomer-17/Gd-DTPA [n = 5]). This study demonstrates that dual-tracer DCE MRI studies yield pharmacokinetic parameters that correlate with immunohistochemical measures of MVD.     

鼻咽癌放疗后脑干损伤的MRI诊断

目的 :提高对鼻咽癌放疗后脑干损伤MRI表现的认识与诊断水平。方法 :分析鼻咽癌放疗后脑干损伤的 17例患者MRI检查资料 ,本组病例在放疗后 0 .5～ 12年出现不同程度颅神经损伤症状。结果 :病变好发于脑干 ,表现为稍长或长T1、长T2 信号。注射Gd DTPA后大多数病例强化。结论 :鼻咽癌放疗后脑干损伤的MRI表现具有一定特点 ,MRI是诊断该病的有效方法。     

Comparison of Gadomer-17 and gadopentetate dimeglumine for differentiation of benign from malignant breast tumors with MR imaging

RATIONALE AND OBJECTIVES: This study compared gadopentetate dimeglumine (molecular weight, 0.5 kD), a standard contrast medium, and Gadomer-17 (apparent molecular weight, approximately 35 kD), a new, clinically applicable, large-molecular contrast medium, with respect to their microvascular characterizations of experimentally induced breast tumors at magnetic resonance (MR) imaging. MATERIALS AND METHODS: A spectrum of breast tumors, benign through highly malignant, was induced in Sprague-Dawley rats (n = 30) by intraperitoneal administration of N-ethyl-N-nitrosourea (ENU), a potent carcinogen. All animals underwent three-dimensional spoiled gradient-recalled MR imaging, with precontrast imaging and dynamic postcontrast imaging after injection of gadopentetate dimeglumine (0.1 mmol/kg) and Gadomer-17 (0.03 mmol/kg), administered in a random order at a 24-hour interval. Several microvascular parameters were compared: the endothelial transfer coefficient (K(PS)), a measure of microvascular permeability; the fractional plasma volume (fPV), and the plasma equivalent volume. Each MR imaging parameter was correlated with histopathologic findings. RESULTS: With Gadomer-17, the mean values for K(PS) and fPV were significantly greater in carcinomas than in fibroadenomas (P < .004 and .04, respectively). With gadopentetate dimeglumine, the mean values for fPV and PEV were significantly greater in carcinomas (P <. 004 and .02, respectively). Because of the high variability within both fibroadenoma and carcinoma groups, however, there were no significant correlations between K(PS), fPV, or PEV and histopathologic tumor grade as indicated by the Scarff-Bloom-Richardson score, for either agent. CONCLUSION: Although the K(PS) and fPV estimates obtained from dynamic MR imaging data with Gadomer-17 enhancement offer some potential for characterizing breast tumors, none of the quantitative microvascular parameters derived with either agent were significantly correlated with histopathologic tumor grade.     

Detection of diffuse glomerular lesions in rats: I. Comparisons of conventional radioactive agents

Conventional renal diagnostic agents, [131I]hippuran, [99mTc]glucoheptonate (GHA), and [99mTc] dimercaptosuccinate (DMS) were compared with [99mTc] or [111In] diethylenetriaminepentaacetic (DTPA) for the detection of glomerular damage in rats compared with controls. The glomerular lesions were induced by the i.v. injection of puromycin aminonucleoside (PA) 9 days before the radionuclide studies, a model of spontaneous "minimal change" glomerulonephritis in humans. Computer-generated early renal uptake of [99mTc]DTPA or GHA correlated with the glomerular filtration rate (GFR) quantitated by biexponential plasma clearance of DTPA administered by single i.v. injection. The early renal uptake of hippuran and DMS correlated poorly with GFR as assessed by DTPA clearance. However, the 2-hr renal retention of DMS correlated well with the DTPA clearance. None of the parameters measured with [131I]hippuran correlated well with DTPA clearance, probably because of decreased protein plasma binding of hippuran secondary to hypoproteinemia in this experimental model. It was concluded that none of these agents was superior to labeled DTPA for the detection of glomerular damage in this experimental model.     

Interstitial magnetic resonance lymphography using a polymeric t1 contrast agent: initial experience with Gadomer-17

RATIONALE AND OBJECTIVES: The detection of lymph node metastases is an important step in tumor staging and is significant for therapy planning. Lymph node-specific contrast agents can raise the sensitivity and specificity of modern diagnostic methods. This study investigated the suitability of the dendritic contrast agent Gadomer-17 in magnetic resonance (MR) lymph node imaging and compared three different dosages in such an application. METHODS: Doses of 1.0, 2.5, and 10.0 micromol Gd/kg body weight were interstitially injected into the hind legs of dogs; the signal intensities of two successive lymph node groups (inguinal and iliacal) were then recorded up to 120 minutes after injection. RESULTS: Gadomer-17 induced a strong increase in signal intensity of the examined lymph node groups. At 15 minutes postinjection, the enhancement increased by 120% to 680%, depending on the dose. The maximum enhancement was 450% to 960% at 60 to 90 minutes postinjection. Doses of 2.5 and 10.0 micromol Gd/kg showed comparable results; even the lowest dose (1.0 micromol Gd/kg) enhanced the contrast of the inguinal lymph nodes in 4 of 5 animals and the iliacal lymph nodes in three of five animals. Therefore, the minimum effective dose of Gadomer-17 in this study was approximately 2.5 micromol Gd/kg. CONCLUSION: This study revealed the excellent suitability of the dendritic contrast agent Gadomer-17 for MR imaging of the lymphatic system (lymph nodes and lymph vessels).     

Clearance of liposomal gadolinium: in vivo decomplexation.

The contrast agents gadolinium-DTPA (diethylenetriaminepentaacetic acid), Gd-DOTA (tetraazacyclododecanetetraacetic acid), and Gd-HP-DO3A (1,4,7-tris[carboxymethyl]-10-[2' hydroxypropyl]-1,4,7,10-tetraazacyclododecane) are used in humans as extracellular contrast agents. Although free Gd+ ion is toxic, the intact Gd3+ complexes are rapidly excreted and are relatively nontoxic. Decomplexation with release of free gadolinium is a relevant clinical concern in patients with altered renal clearance. Blood pool contrast agents currently under development may have longer clearance half-lives and be more prone to decomplexation. The present study was designed to evaluate the clearance of liposomally encapsulated Gd3+ complexes (DTPA, DOTA, and HP-DO3A). The macrocyclic compounds had more rapid and complete clearance than DTPA (P less than .05). Parallel studies with carbon-14 and Gd-153-labeled complexes showed significant differences (P less than .05) in the amount of these isotopes retained in the heart, kidney, lungs, and spleen, providing strong supportive evidence for in vivo decomplexation.     

AIDS合并脑内、肺内机遇性感染的影象学诊断研究

目的：研究AIDS合并脑内、肺内机遇性感染的影像表现及其与AIDS确诊的相关性。方法：101例AIDS患中合并脑内(22例)和肺内(24例)机遇性感染均经MRI检查，部分病人还经Gd-DTPA增强及CT扫描。结果：在一些典型的合并脑内机遇性感染患中，CT平扫显示脑实质内多发低密度区，增强后病变呈异常强化、MRI显示两侧大脑自质广泛长T1、长T2异常信号，个别病人伴少量出血，Gd-DTPA增强后，脑实质内病变呈多发环状、螺旋形明显异常强化。在典型的合并肺内机遇性感染患中，CT示右肺大片状高密度区、左侧胸腔内积液。结论：AIDS合并脑内、肺内机遇性感染的影像表现无特异性征象，确诊需靠血清HIV检验。     

Gd-DTPA增强的MRI在脑转移瘤诊断中的应用

目的探讨Gd-DTPA增强MRI对脑转移瘤的诊断价值及意义。方法回顾分析100例脑部转移瘤病人的平扫和增强MRI资料。结果100例脑转移瘤病人,平扫检出198个转移病灶,增强扫描检出354个转移病灶。平扫检出坏死囊变病灶85个,增强扫描检出183个。平扫检出病灶最小直径0.8cm,增强扫描检出病灶最小直径0.3cm,98例增强后可清晰显示病灶边界。16例没有症状的病人平扫7例未显示病灶,增强后均发现有转移病灶。结论Gd-DTPA是一种安全的MRI对比剂,增强MRI在显示脑转移瘤的大小、数目、边界、内部结构及位置等方面具有明显优势,可以帮助早期诊断。     

Simultaneous MRI measurement of blood flow, blood volume, and capillary permeability in mammary tumors using two different contrast agents

A technique for the simultaneous measurement of three vascular parameters: blood flow (Frho), blood volume (v(b)), and the capillary permeability-surface area product (PSrho) in breast tumors using dynamic contrast-enhanced magnetic resonance imaging (MRI) is presented. Features of the technique include measurement of precontrast tumor T(1), rapid temporal sampling, measurement of the arterial input function, and use of a distributed parameter tracer kinetic model. Parameter measurements are compared that were determined using two contrast agents of different molecular weights, gadolinium-diethylene triamine pentaacetic acid (Gd-DTPA; 0.6 kDa) and Gadomer-17 (17 kDa), in 18 spontaneous canine mammary tumors. Measurements of Frho and v(b) corresponded well with literature values, and the mean PSrho measured using Gd-DTPA was a factor of 15 higher than that measured using Gadomer-17. J. Magn. Reson. Imaging 2000;12:991-1003.     

Comparison of different radioactive renal agents in cisplatin-induced tubular toxicity in rats

The efficacy of five different radiodiagnostic agents for detecting renal tubular dysfunction induced with cisplatin in rats was compared to controls. Diethylenetriaminepentaacetic acid (DTPA) labeled with 99mTc or 111In was administered simultaneously with each of the other four agents [99mTc]glucoheptonate, [99mTc]dimercaptosuccinic acid, [131I]hippuran and [111In]lysozyme) as a standard to normalize for differences in functional impairment from animal to animal from the same dose of cisplatin. The 2-hr plasma clearance and computer-generated 2- to 3-min uptake in the two kidneys with [99mTc]dimercaptosuccinic acid were significantly inferior to similar measurements with the other agents in differentiating abnormal from normal function. The 2-hr uptake of [99mTc]glucoheptonate and [111In]lysozyme proved of no value in this differentiation. The late renal retention of [99mTc]dimercaptosuccinic acid well separated the cisplatin from control rats, but the greatest difference was observed by the 2-hr uptakes of [131I]hippuran and DTPA.     

Evaluation of cyclosporine nephrotoxicity in rats with various renal radioactive agents

The efficacy of different radiodiagnostic agents for demonstrating the decline in renal function from cyclosporine (CyA) nephrotoxicity was assessed in rats receiving a standard dose of the drug for 2 wk, compared with control rats. The agents included [99mTc]DTPA, [131I]hippuran, [111In]lysozyme, [99mTc]glucoheptonate (GHA), [99mTc]dimercaptosuccinate (DMS) and [111In]aminated dextran (amdex). A small dose of [99mTc]- or [111In]DTPA was administered simultaneously to normalize the results for variations in drug response from one animal to another. There were statistically significant differences in the detectability of the renal functional impairment by plasma clearance, early and 2-hr renal uptake among the different agents. However, none was clearly superior to DTPA. This conclusion is consistent with previous studies which showed a parallel decline in glomerular filtration rate (GFR) and effective renal plasma flow in acute CyA toxicity probably due primarily to vasoconstriction.     

Dynamic contrast-enhanced MR imaging of bacterial abscess and VX2 carcinoma in rabbits: comparison of gadopentetate dimeglumine and a macromolecular contrast agent

OBJECTIVE: The objective of this study was to compare enhancement patterns of a blood-pool contrast agent, Gadomer-17, with those of gadopentetate dimeglumine in bacterial abscesses and VX2 carcinoma in rabbits. MATERIALS AND METHODS: Fourteen rabbits with experimentally induced bacterial abscesses and VX2 carcinoma in both thighs underwent dynamic contrast-enhanced MR imaging with Gadomer-17 and gadopentetate dimeglumine at a 24-hr interval. The enhancement ratios (postcontrast to precontrast signal intensities) of lesions in the same animal were assessed and correlated with microvessel density. RESULTS: For Gadomer-17, the enhancement ratio of the abscesses (1.66 +/- 0.39) peaked 15 min after the injection, while that of the carcinoma (2.05 +/- 0.16) peaked at 10 min. The enhancement ratios of the carcinoma were consistently higher than those of the abscesses up to 30 min. For gadopentetate dimeglumine, peak enhancement ratio of the abscesses (2.30 +/- 0.75) was seen 5 min after the injection, while that of the carcinoma (2.32 +/- 0.51) was seen at 3 min. The enhancement ratios of the carcinomas were significantly higher at 1 min, but significantly lower at 20-30 min, compared with those of the abscesses, as a result of rapid decrease of enhancement ratios in the carcinomas. The microvessel density was 9.8 +/- 5.2 vessels per field of view for the abscesses and 36.3 +/- 9.5 vessels per field of view for the carcinoma (p < 0.001). CONCLUSION: Delayed peak enhancement and slow decay were found in both bacterial abscess and VX2 carcinoma with Gadomer-17, whereas early peak enhancement and rapid decay were found especially in VX2 carcinoma with gadopentetate dimeglumine. Enhancement ratios on MR imaging with a blood-pool contrast agent correlated well with the microvessel density in bacterial abscess and VX2 carcinoma.     

Detection of diffuse glomerular lesions in rats: II. Comparison of indium-111 cationic small macromolecules with technetium-99m DTPA

Dextrans with average molecular weights of 5,000, 10,000, and 17,500 and inulin were rendered cationic by amination with 2-bromoethylamine hydrobromide. After limited coupling with DTPA cyclic dianhydride, they were labeled with 111In. A good correlation was found between their early renal uptake quantitated by camera-computer techniques and their renal clearance from multiple plasma samples in rats with glomerular damage induced by puromycin aminonucleoside and controls. However, there was poor correlation between the early renal uptake of these agents and the clearance of simultaneously injected [99mTc]DTPA. The 2-hr organ distribution and urinary excretion of these agents were compared with the corresponding values of DTPA. The differences in clearance between rats with glomerular damage and controls were greater with aminated dextran (mol wt 5,000) than with DTPA, confirming previous work with infusions of nonradioactive charged dextrans and neutral inulin. The cationic dextrans appear to reflect the presence or absence of the normal anionic charge of the glomerular membrane as well as changes in filtration rate. Aminated inulin did not differentiate between controls and rats with glomerular disease any better than DTPA, probably because the number of amino groups conjugated was insufficient to produce the charge effect.     

Micro-MR angiography of normal and intratumoral vessels in mice using dedicated intravascular MR contrast agents with high generation of polyamidoamine dendrimer core: reference to pharmacokinetic properties of dendrimer-based MR contrast agents.

Pharmacokinetic characteristics of intravascular macromolecular magnetic resonance imaging (MRI) contrast agents with polyamidoamine dendrimer cores smaller than generation-7 were previously studied in the literature. To evaluate the effects of greater hepatic uptake on the pharmacokinetics of the larger generation dendrimers, the MRI contrast agents GxD-(1B4M-Gd)(2(x+2)) were synthesized with generation-7, -8, and -9 polyamidoamine dendrimers and 2-(p-isothiocyanatobenzyl)-6-methyl-diethylenetriaminepentaacetic acid (1B4M). Their pharmacokinetic characteristics in mice were compared with that of G6D-(1B4M-Gd)(256). In biodistribution and dynamic micro-MRI studies, significantly less renal accumulation of G7D-(1B4M-Gd)(512), G8D-(1B4M-Gd)(1024), and G9D-(1B4M-Gd)(2048) was shown compared to G6D-(1B4M-Gd)(256) (P < 0.01). There was a significantly greater accumulation of G8D-(1B4M-Gd)(1024) and G9D-(1B4M-Gd)(2048) in the liver compared to G6D-(1B4M-Gd)(256) and G7D-(1B4M-Gd)(512) (P < 0.01). The highest blood retention of all dendrimer-based MRI contrast agents was exhibited by G7D-(1B4M-Gd)(512) (P < 0.01). The normal and intratumoral fine vessels of approximately 100 microm diameter were visualized in normal or tumor-bearing mice by high resolution three-dimensional-micro-MR angiographs with G7D-(1B4M-Gd)(512) and G8D-(1B4M-Gd)(1024) with good vessel-to-soft tissue contrast. In summary, increased accumulation in the liver with concomitant decreased uptake in the kidney was caused by increased molecular sizes of the dendrimer-based MRI contrast agents.     

Enhanced tissue differentiation in the developing mouse brain using magnetic resonance micro‐histology

Purpose: Worldwide efforts to understand developmental processes demand new high‐resolution 3D imaging methods to detect the consequences of gene function in embryo development and diseases. Encouragingly, recent studies have shown that MRI contrast agents can highlight specific tissue structures in ex vivo adult mouse brains. MR imaging of mouse embryos is currently limited by a lack of tissue staining capabilities that would provide the flexibility and specificity offered by histological stains conventionally used for mouse embryo phenotyping. Methods: The MRI staining properties of two readily available contrast agents, Mn‐DPDP and Gd‐DTPA, were investigated in mid‐gestation mouse embryos. Results: Brain tissue substructures not normally visible using MRI were detected. Mn‐DPDP and Gd‐DTPA provided spatially distinct tissue staining patterns. An initial assessment indicated that these agents utilized independent contrast enhancement mechanisms. Mn‐DPDP was identified as a potential MRI contrast agent for enhancement of mouse embryonic cellular density and enabled identification of regions containing populations of neural stem and progenitor cells within the intact embryo brain. Conclusions: Different contrast agents may be used to provide tissue‐specific contrast enhancement, suggesting that a host of specialized MRI stains may be available for probing the developing mouse brain and investigating developmental and disease mechanisms. Magn Reson Med 70:1380–1388, 2013. © 2012 Wiley Periodicals, Inc.     

Positive effects of polyethylene glycol conjugation to generation-4 polyamidoamine dendrimers as macromolecular MR contrast agents.

Macromolecules conjugated with polyethylene glycol (PEG) acquire more hydrophilicity, resulting in a longer half-life in circulation and lower immunogenicity. Two novel conjugates for MRI contrast agents were synthesized from a generation-4 polyamidoamine dendrimer (G4D), 2-(p-isothiocyanatobenzyl)-6-methyl-diethylenetriaminepentaacetic acid (1B4M), and one or two PEG molecules with a molecular weight of 20000 Da (PEG(2)-G4D-(1B4M-Gd)(62) (MW: 96 kD), PEG(1)-G4D-(1B4M-Gd)(63) (MW: 77 kD)). Their pharmacokinetics, excretion, and properties as vascular MRI contrast agents were evaluated and compared with those of G4D-(1B4M-Gd)(64) (MW: 57 kD). PEG(2)-G4D-(1B4M-Gd)(62) remained in the blood significantly longer and accumulated significantly less in the liver and kidney than the other two preparations (P < 0.01). Although the blood clearance was slower, PEG(2)-G4D-(1B4M-Gd)(62) was excreted more readily without renal retention than the other two preparations. In conclusion, the positive effects of PEG conjugation on a macromolecular MRI contrast agent were found to be prolonged retention in the circulation, increased excretion, and decreased accumulation in the organs.     

PAMAM dendrimer-based contrast agents for MR imaging of Her-2/neu receptors by a three-step pretargeting approach.

Pretargeting of receptors is a useful approach in molecular imaging and therapy to reduce background noise or toxicity and enhance selectivity. In this study a three-step pretargeting approach that includes a biotinylated antibody, avidin/streptavidin, and a biotinylated imaging agent is described. A PAMAM dendrimer generation 4 (G4D)-based MRI T(1) agent biotin-G4D-DTPA-Gd (bG4D-Gd) and its sister compound with remaining free surface amine groups blocked by succinic anhydride to reduce positive charges (bG4D-Gd-SA) were synthesized. Limited selective enhancement in MRI was observed in a Her-2/neu mouse tumor xenograft by this three-step pretargeting approach that includes biotinylated trastuzumab, avidin and bG4D-Gd, or bG4D-Gd-SA. However, these dendrimer-based MRI agents with molecular weight around 29 kD reached and remained in the tumor through the enhanced permeability and retention effect. Prolonged and extensive accumulation of both bG4D-Gd and b-G4-Gd-SA in the kidneys was also observed.