MR Molecular Imaging of Prostate Cancer with a Small Molecular CLT1 Peptide Targeted Contrast Agent

Tumor extracellular matrix has abundance of cancer related proteins that can be used as biomarkers for cancer molecular imaging. In this work, we demonstrated effective MR cancer molecular imaging with a small molecular peptide targeted Gd-DOTA monoamide complex as a targeted MRI contrast agent specific to clotted plasma proteins in tumor stroma. We performed the experiment of evaluating the effectiveness of the agent for non-invasive detection of prostate tumor with MRI in a mouse orthotopic PC-3 prostate cancer model. The targeted contrast agent was effective to produce significant tumor contrast enhancement at a low dose of 0.03 mmol Gd/kg. The peptide targeted MRI contrast agent is promising for MR molecular imaging of prostate tumor.     

Synthesis and characterization of a new bioactivated paramagnetic gadolinium(III) complex [Gd(DOTA-FPG)(H2O)] for tracing gene expression

A smart contrast agent for magnetic resonance imaging (MRI) can be used to exploit an enzymatic activity specific to the tissue or disease state signified by converting an MRI-inactivated agent to an activated MRI agent. In this study, a beta-galactopyranose-containing gadolinium(III) complex [Gd(DOTA-FPG)(H 2O)] was designed, synthesized, and characterized as being potentially suitable for a bioactivated MRI contrast agent. The (17)O NMR experiments were conducted to estimate the water exchange rate k e x 298 and rotational correlation time tau R 298 . The k ex 298 value of [Gd(DOTA-FPG)(H 2O)] is similar to that of [Gd(DO3A-bz-NO 2)(H 2O)]. The rotational correlation time value of [Gd(DOTA-FPG)(H 2O)] is dramatically longer than that of [Gd(DOTA)(H 2O)] (-) Relaxometric studies show that the percentage change in the T 1 value of [Gd(DOTA-FPG)(H 2O)] decreases dramatically in the presence of beta-galactosidase and human serum albumin. The T(1) change percentage of [Gd(DOTA-FPG)(H 2O)] (60%) is significantly higher than those of Egad and gadolinium(III)-1-(4-(2-(1-(4,7,10-triscarboxymethyl-(1,4,7,10-tetraazacyclododecyl)))-ethylcarbamoyloxymethyl)-2-nitrophenyl)-beta- d-glucopyronuronate. The signal intensity of the MR image for [Gd(DOTA-FPG)(H 2O)] in the presence of human serum albumin and beta-galactosidase (2670 +/- 210) is significantly higher than that of [Gd(DOTA-FPG)(H 2O)] in the sodium phosphate buffer solution (1490 +/- 160). In addition, the MR images show a higher-intensity enhancement in CT26/beta-gal tumor with beta-galactosidase gene expression but not for the CT26 tumor without beta-galactosidase gene expression. We conclude that [Gd(DOTA-FPG)(H 2O)] is a suitable candidate for a bioactivated MRI contrast agent in tracing gene expression.     

Targeted gadolinium-loaded dendrimer nanoparticles for tumor-specific magnetic resonance contrast enhancement

A target-specific MRI contrast agent for tumor cells expressing high affinity folate receptor was synthesized using generation five (G5) ofpolyamidoamine (PAMAM) dendrimer. Surface modified dendrimer was functionalized for targeting with folic acid (FA) and the remaining terminal primary amines of the dendrimer were conjugated with the bifunctional NCS-DOTA chelator that forms stable complexes with gadolinium (Gd III). Dendrimer-DOTA conjugates were then complexed with GdCl3 followed by ICP-OES as well as MRI measurement of their longitudinal relaxivity (T1 s(-1) mM(-1)) of water. In xenograft tumors established in immunodeficient (SCID) mice with KB human epithelial cancer cells expressing folate receptor (FAR), the 3D MRI results showed specific and statistically significant signal enhancement in tumors generated with targeted Gd(III)-DOTA-G5-FA compared with signal generated by non-targeted Gd(III)-DOTA-G5 contrast nanoparticle. The targeted dendrimer contrast nanoparticles infiltrated tumor and were retained in tumor cells up to 48 hours post-injection of targeted contrast nanoparticle. The presence of folic acid on the dendrimer resulted in specific delivery of the nanoparticle to tissues and xenograft tumor cells expressing folate receptor in vivo. We present the specificity of the dendrimer nanoparticles for targeted cancer imaging with the prolonged clearance time compared with the current clinically approved gadodiamide (Omniscan) contrast agent. Potential application of this approach may include determination of the folate receptor status of tumors and monitoring of drug therapy.     

Synthesis and evaluation of globular Gd-DOTA-monoamide conjugates with precisely controlled nanosizes for magnetic resonance angiography

The purpose of this study was to design and prepare macromolecular contrast agents (CAs) with a precisely defined globular structure for MR angiography and tumor angiogenesis imaging. Generations 1 through 3 (Gd-DOTA-monoamide)-poly-L-lysine octasilsesquioxane dendrimers were prepared as nanoglobular MRI CAs. The nanoglobular Gd(III) chelates had a well-defined compact globular structure and high loading of Gd-DOTA-monoamide at their surface. The size of the G1, G2, and G3 nanoglobular MRI CAs was approximately 2.0, 2.4, and 3.2 nm, respectively. The T1 relaxivity of G1, G2, and G3 nanoglobular MRI CAs was approximately 6.4, 7.2, and 10.0 mM(-1) sec(-1) at 3T, respectively. The nanoglobular MRI CAs showed size-dependent contrast enhancement within the mouse vasculature, which gradually decayed to baseline after a 60 min session. The G3 nanoglobular CA resulted in more significant and prolonged vascular enhancement than the smaller nanoglobular agents at 0.03 mmol Gd/kg. The G3 agent also provided significant and prolonged contrast enhancement in the heart and vasculature at a dose as low as 0.01 mmol Gd/kg, 1/10th of the regular clinical dose. Significant enhancement was observed in tumor for all CAs. The nanoglobular CAs cleared via renal filtration and accumulated in the urinary bladder as shown in the dynamic MR images. The nanoglobular Gd(III) chelates are effective intravascular MRI CAs at substantially reduced doses. The nanoglobular MRI CAs are promising for further preclinical development for MR angiography and MR imaging of tumor angiogenesis.     

Design,synthesis, and in vitro evaluation of a binary targeting MRI contrast agent for imaging tumor cells

A binary targeting vector that consists of peptide sequences of Arg-Gly-Asp (RGD) and Asn-Gly-Arg (NGR) motifs has been designed and synthesized using solid-phase peptide synthesis procedure. The vector is then coupled with Gd-DOTA to work as a targeting contrast agent (CA1) for magnetic resonance imaging of human lung adenocarcinoma cells A549. Its longitudinal relaxivity is measured to be 7.55 mM^?1 s^?1 in aqueous solution at a magnetic field of 11.7 T, which is higher than that of Magnevist (4.25 mM^?1 s^?1) in the same conditions. The cell experiment shows, at the same concentration, uptake quantity of CA1 by A549 is much more than Magnevist and also superior over CA2 (a single targeting contrast agent contains only RGD). The uptake can be blocked by the targetable peptide containing RGD or NGR without coupling Gd. To summarize, CA1 has very good ability to target A549 and higher relaxivity than that of Magnevist. So CA1 is promising MRI contrast agent for high-resolution MR molecular imaging of human lung adenocarcinoma A549 cells.     

RGD targeted poly(L-glutamic acid)-cystamine-(Gd-DO3A) conjugate for detecting angiogenesis biomarker alpha(v) beta3 integrin with MRT, mapping

Cyclic Arg-Gly-Asp-D-Phe-Lys [c(RGDfK)] targeted poly(L-glutamic acid) (PGA)-(Gd-DO3A) conjugate with a biodegradable cystamine spacer was prepared and evaluated for in vivo detection of an angiogenesis biomarker, alpha(v)beta3 integrin, in neoplastic tissues with T1 mapping, a quantitative magnetic resonance imaging (MRI) technique. The binding activity of the c(RGDfK) containing conjugate was investigated using in vitro vitronectin assay with human prostate carcinoma DU145 cell line and Kaposi's sarcoma SLK cell line. The peptide c(RGDfK) and PGA-cystamine-(Gd-DO3A) conjugate were used as controls. The binding affinity of polymer bound c(RGDfK) was slightly lower than free c(RGDfK) peptide. The RGD targeted conjugate had higher binding affinity to the DU145 cells than the SLK cells, which was consistent to free c(RGDfK). The imaging of alpha(v)beta3 integrin with targeted PGA-cystamine-(Gd-DO3A) was evaluated in nude mice bearing DU145 and SLK xenografts at a dose of 5 micromol-Gd/kg. The targeted conjugate demonstrated higher in vivo binding affinity to the DU145 xenografts than the SLK xenografts, resulting in a significant decrease of T1 values of water protons in the periphery of the DU145 tumors as shown in the MR T1 maps. No significant decrease of T1 values was observed in the SLK tumor with the targeted conjugate and in both tumors with the non-targeted conjugate. The targeted polymeric Gd(III) chelate conjugate with a degradable spacer has the potential to be a new paradigm for safe and effective probes in molecular imaging with quantitative MR T1 mapping.     

Design of (Gd-DO3A)n-polydiamidopropanoyl-peptide nucleic acid-D(Cys-Ser-Lys-Cys) magnetic resonance contrast agents

We hypothesized that chelating Gd(III) to 1,4,7-tris(carboxymethylaza)cyclododecane-10-azaacetylamide (DO3A) on peptide nucleic acid (PNA) hybridization probes would provide a magnetic resonance genetic imaging agent capable of hybridization to a specific mRNA. Because of the low sensitivity of Gd(III) as an magnetic resonance imaging (MRI) contrast agent, a single Gd-DO3A complex per PNA hybridization agent could not provide enough contrast for detection of cancer gene mRNAs, even at thousands of mRNA copies per cell. To increase the Gd(III) shift intensity of MRI genetic imaging agents, we extended a novel DO3An-polydiamidopropanoyl (PDAPm) dendrimer, up to n = 16, from the N-terminus of KRAS PNA hybridization agents by solid phase synthesis. A C-terminal D(Cys-Ser-Lys-Cys) cyclized peptide analog of insulin-like growth factor 1 (IGF1) was included to enable receptor-mediated cellular uptake. Molecular dynamic simulation of the (Gd-DO3A-AEEA)16-PDAP4-AEEA2-KRAS PNA-AEEA-D(Cys-Ser-Lys-Cys) genetic imaging nanoparticles in explicit water yielded a pair correlation function similar to that of PAMAM dendrimers, and a predicted structure in which the PDAP dendron did not sequester the PNA. Thermal melting measurements indicated that the size of the PDAP dendron included in the (DO3A-AEEA)n-PDAPm-AEEA2-KRAS PNA-AEEA-D(Cys-Ser-Lys-Cys) probes (up to 16 Gd(III) cations per PNA) did not depress the melting temperatures (Tm) of the complementary PNA/RNA hybrid duplexes. The Gd(III) dendrimer PNA genetic imaging agents in phantom solutions displayed significantly greater T1 relaxivity per probe (r1 = 30.64 +/- 2.68 mM(-1) s(-1) for n = 2, r1 = 153.84 +/- 11.28 mM(-1) s(-1) for n = 8) than Gd-DTPA (r1 = 10.35 +/- 0.37 mM(-1) s(-1)), but less than that of (Gd-DO3A)32-PAMAM dendrimer (r1 = 771.84 +/- 20.48 mM(-1) s(-1)) (P < 0.05). Higher generations of PDAP dendrimers with 32 or more Gd-DO3A residues attached to PNA-D(Cys-Ser-Lys-Cys) genetic imaging agents might provide greater contrast for more sensitive detection.     

Chlorophyll-a analogues conjugated with aminobenzyl-DTPA as potential bifunctional agents for magnetic resonance imaging and photodynamic therapy

A clinically relevant photosensitizer, 3-devinyl-3-(1-hexyloxyethyl)pyropheophorbide-a (HPPH, a chlorophyll-a derivative), was conjugated with Gd(III)-aminobenzyl-diethylenetriaminepentaacetic acid (DTPA), an experimental magnetic resonance (MR) imaging agent. In vivo reflectance spectroscopy confirmed tumor uptake of HPPH-aminobenzyl-Gd(III)-DTPA conjugate was higher than free HPPH administered intraveneously (iv) to C3H mice with subcutaneously (sc) implanted radiation-induced fibrosarcoma (RIF) tumor cells. In other experiments, Sprague-Dawley (SD) rats with sc implanted Ward Colon Carcinoma cells yielded markedly increased MR signal intensities from tumor regions-of-interest (ROIs) 24 h post-iv injection of HPPH-aminobenzyl-Gd(III)-DTPA conjugate as compared to unconjugated HPPH. In both in vitro (RIF tumor cells) and in vivo (mice bearing RIF tumors and rats bearing Ward Colon tumors) the conjugate produced significant increases in tumor conspicuity at 1.5 T and retained therapeutic efficacy following PDT. Also synthesized were a series of novel bifunctional agents containing two Gd(III) atoms per HPPH molecule that remained tumor-avid and PDT-active and yielded improved MR tumor conspicuity compared to their corresponding mono-Gd(III) analogues. Administered iv at a MR imaging dose of 10 micromol/kg, these conjugates produced severe skin phototoxicity. However, by replacing the hexyl group of the pyropheophorbide-a with a tri(ethylene glycol) monomethyl ether (PEG-methyl ether), these conjugates produced remarkable MR tumor enhancement at 8 h post-iv injection, significant tumoricidal activity (80% of mice were tumor-free on day 90), and reduced skin phototoxicity compared to their corresponding hexyl ether analogues. The poor water-solubility characteristic of these conjugates was resolved by incorporation into a liposomal formulation. This paper presents the synthesis of tumor-avid contrast enhancing agents for MR imaging and thus represents an important milestone toward improving cancer diagnosis and tumor characterization. More importantly, this paper describes a new family of bifunctional agents that combine two modalities into a single cost-effective "see and treat" approach, namely, a single agent that can be used for contrast agent-enhanced MR imaging followed by targeted photodynamic therapy.     

Molecular Magnetic Resonance Imaging of Tumors with a PTPµ Targeted Contrast Agent

Molecular magnetic resonance imaging (MRI) of tumors improves the specificity of MRI by using targeted probes conjugated to contrast-generating metals. The limitation of this approach is in the identification of a target molecule present in sufficient concentration for visualization and the development of a labeling reagent that can penetrate tumor tissue with the fast kinetics required for use in a clinical setting. The receptor protein tyrosine phosphatase PTPµ is a transmembrane protein that is continuously proteolyzed in the tumor microenvironment to generate a high concentration of extracellular fragment that can be recognized by the SBK2 probe. We conjugated the SBK2 peptide to a gadolinium chelate [SBK2-Tris-(Gd-DOTA)₃] to test whether the SBK2 probe could be developed as an MR molecular imaging probe. When intravenously injected into mice bearing flank tumors of human glioma cells, SBK2-Tris-(Gd-DOTA)₃ labeled the tumors within 5 minutes with a high level of contrast for up to 2 hours post-injection. The contrast enhancement of SBK2-Tris-(Gd-DOTA)₃ was significantly higher than that observed with a current MRI macrocyclic gadolinium chelate (Gadoteridol, ProHance) alone or a scrambled control. These results demonstrate that SBK2-Tris-(Gd-DOTA)₃ labeling of the PTPµ extracellular fragment is a more specific MR molecular imaging probe than ProHance or a scrambled control. Consequently, the SBK2 probe may be more useful than the current gold standard reagent for MRI to identify tumors and to co-register tumor borders during surgical resection.     

Comparison between GdDTPA and two gadolinium polyoxometalates as potential MRI contrast agents

Two gadolinium polyoxometalates, Gd(2)P(2)W(18)O(62) and K(15)[(GdO)(3)(PW(9)O(34))(2)], have been evaluated by in vivo as well as in vitro experiments as the candidates of tissue-specific magnetic resonance imaging (MRI) contrast agents. T(1)-relaxivities of 28.4 mM(-1).s(-1) for Gd(2)P(2)W(18)O(62) and 11.2 mM(-1).s(-1) for K(15)[(GdO)(3)(PW(9)O(34))(2)] (400 MHz, 25 degrees C) were higher than that of the commercial MRI contrast agent (GdDTPA). Their relaxivities in bovine serum albumin and human serum transferrin were also reported. The favorable liver-specific contrast enhancement and renal excretion capability in in vivo MRI with Sprague-Dawley rats after i.v. administration of K(15)[(GdO)(3)(PW(9)O(34))(2)] was demonstrated. In vivo and in vitro assay showed that K(15)[(GdO)(3)(PW(9)O(34))(2)] is a promising liver-specific MRI contrast agent. However, Gd(2)P(2)W(18)O(62) did not show the favorable quality in vivo as expected from its high relaxivity in vitro, which was attributed to low bioavailability, indicating that it is of limited value as tissue-specific MRI contrast agent.     

HER2 targeted molecular MR imaging using a de novo designed protein contrast agent

The application of magnetic resonance imaging (MRI) to non-invasively assess disease biomarkers has been hampered by the lack of desired contrast agents with high relaxivity, targeting capability, and optimized pharmacokinetics. We have developed a novel MR imaging probe targeting to HER2, a biomarker for various cancer types and a drug target for anti-cancer therapies. This multimodal HER20targeted MR imaging probe integrates a de novo designed protein contrast agent with a high affinity HER2 affibody and a near IR fluorescent dye. Our probe can differentially monitor tumors with different expression levels of HER2 in both human cell lines and xenograft mice models. In addition to its 100-fold higher dose efficiency compared to clinically approved non-targeting contrast agent DTPA, our developed agent also exhibits advantages in crossing the endothelial boundary, tissue distribution, and tumor tissue retention over reported contrast agents as demonstrated by even distribution of the imaging probe across the entire tumor mass. This contrast agent will provide a powerful tool for quantitative assessment of molecular markers, and improved resolution for diagnosis, prognosis and drug discovery.     

A new class of Gd-based DO3A-ethylamine-derived targeted contrast agents for MR and optical imaging

Synthetic bifunctional probes based on [4,7-bis-carboxymethyl-10-(2-aminoethyl)-1,4,7,10-tetraaza-cyclododec-1-yl]-acetic acid (DO3A-ethylamine) preloaded with gadolinium were prepared for applications in targeted magnetic resonance imaging (MRI) and optical imaging. A convenient route of synthesis is reported, which allowed conjugation of this probe with biomolecules for the preparation of model MR contrast agents for targeted imaging. The conjugated probes have the following interesting properties: GdDO3A-ethylamido-biotin (Gd-9) can be used for targeted imaging using an avidin-biotin system. The fluorescent probe GdDO3A-ethylthiourea-fluorescein (Gd-12) is a bimodal compound, which can be used for both MR and optical imaging. The precursors, DO3A-ethylamidopropyl-maleimide and DO3A-ethyl-isothiocyanate contain a highly reactive moiety, which can interact with free SH-terminals and N-terminals of biological molecules, respectively. In vitro MR relaxivity studies were performed at 300 MHz using different concentrations and chemical environments. MR relaxivity for ligand Gd-9 at pH 7.4, r1 was (3.32 +/- 0.03) s(-1) mM(-1) and r2 was (5.02 +/- 0.14) s(-1) mM(-1). For the mixture of Gd-9 with avidin, at pH 7.4, relaxivity increased linearly with the avidin concentration. A relaxivity enhancement of 45% for r1 and more than 400% for r2 with respect to the unbound biotinylated Gd3+ complex was found at a ratio of 4:1. MR relaxivity for ligand Gd-12, r1 was (5.36 +/- 0.05) s(-1) mM(-1) at pH 7.4. Fluorescence microscopy and spectroscopy of Gd-12-labeled 3T3 mouse fibroblasts showed a concentration-dependent intracellular uptake, accompanied by a slight dose-dependent increase in toxicity up to 150 microM. MR studies on labeled cells indicated a contrast enhancement in both T1- and T2-weighted images by the internalized compound, with the effect being more pronounced in T2-weighted images. Our results indicate that DO3A-ethylamine is a multipurpose precursor, from which various targeted contrast agents can be synthesized after a single-step conjugation with organic/bioorganic molecules.     

A novel polyacrylamide magnetic nanoparticle contrast agent for molecular imaging using MRI

A novel polyacrylamide superparamagnetic iron oxide nanoparticle platform is described which has been synthetically prepared such that multiple crystals of iron oxide are encapsulated within a single polyacrylamide matrix (PolyAcrylamide Magnetic [PAM] nanoparticles). This formulation provides for an extremely large T2 and T2* relaxivity of between 620 and 1140 sec(-1) mM(-1). Administration of PAM nanoparticles into rats bearing orthotopic 9L gliomas allowed quantitative pharmacokinetic analysis of the uptake of nanoparticles in the vasculature, brain, and glioma. Addition of polyethylene glycol of varying sizes (0.6, 2, and 10 kDa) to the surface of the PAM nanoparticles resulted in an increase in plasma half-life and affected tumor uptake and retention of the nanoparticles as quantified by changes in tissue contrast using MRI. The flexible formulation of these nanoparticles suggests that future modifications could be accomplished allowing for their use as a targeted molecular imaging contrast agent and/or therapeutic platform for multiple indications.     

Prostate‐specific membrane antigen‐targeted photoacoustic imaging of prostate cancer in vivo

A sensitive, noninvasive method to detect localized prostate cancer, particularly for early detection and repetitive study in patients undergoing active surveillance, remains an unmet need. Here, we propose a molecular photoacoustic (PA) imaging approach by targeting the prostate‐specific membrane antigen (PSMA), which is over‐expressed in the vast majority of prostate cancers. We performed spectroscopic PA imaging in an experimental model of prostate cancer, namely, in immunocompromised mice bearing PSMA+ (PC3 PIP) and PSMA? (PC3 flu) tumors through administration of the known PSMA‐targeted fluorescence agent, YC‐27. Differences in contrast between PSMA+ and isogenic control tumors were observed upon PA imaging, with PSMA+ tumors showing higher contrast in average of 66.07‐fold with 5 mice at the 24‐hour postinjection time points. These results were corroborated using standard near‐infrared fluorescence imaging with YC‐27, and the squared correlation between PA and fluorescence intensities was 0.89. Spectroscopic PA imaging is a new molecular imaging modality with sufficient sensitivity for targeting PSMA in vivo, demonstrating the potential applications for other saturable targets relevant to cancer and other disorders.      

Conjugation of a water-soluble gadolinium endohedral fulleride with an antibody as a magnetic resonance imaging contrast agent

Water-soluble gadofullerides exhibited high efficiency as magnetic resonance imaging (MRI) contrast agents. In this paper, we report the conjugation of the newly synthesized gadofulleride, Gd@C82O6(OH) 16(-)(NHCH2CH2COOH)8, with the antibody of green fluorescence protein (anti-GFP), as a model for "tumor targeted" imaging agents based on endohedral metallofullerenes. In this model system, the activity of the anti-GFP conjugate can be conveniently detected by green fluorescence protein (GFP), leading to in vitro experiments more direct and facile than those of tumor antibodies. Objective-type total internal reflection fluorescence microscopy revealed that each gadofulleride aggregate conjugated on average five anti-GFPs, and the activity of anti-GFPs was preserved after conjugation. In addition, the gadofulleride/antibody conjugate exhibited higher water proton relaxivity (12.0 mM (-1) s (-1)) than the parent gadofulleride aggregate (8.1 mM (-1) s (-1)) in phosphate buffered saline at 0.35 T, as also confirmed by T1-weighted images of phantoms. These observations clearly indicate that the synthesized gadofulleride/antibody conjugate not only has targeting potential, but also exhibits higher efficiency as an MRI contrast agent.     

In Vivo evaluation of a PAMAM-cystamine-(Gd-DO3A) conjugate as a biodegradable macromolecular MRI contrast agent

Macromolecular Gd(III) chelates are superior magnetic resonance imaging (MRI) contrast agents for blood pool and tumor imaging. However, their clinical development is limited by the safety concerns related to the slow excretion and long-term gadolinium tissue accumulation. A generation 6 PAMAM Gd(III) chelate conjugate with a cleavable disulfide spacer, PAMAM-G6-cystamine-(Gd-DO3A), was prepared as a biodegradable macromolecular MRI contrast agent with rapid excretion from the body. T(1) and T(2) relaxivities of the contrast agent were 11.6 and 13.3 mM(-1)sec(-1) at 3T, respectively. Blood pool and tumor contrast enhancement of the agent were evaluated in female nude mice bearing MDA-MB-231 human breast carcinoma xenografts with a nondegradable conjugate PAMAM-G6-(Gd-DO3A) as a control. PAMAM-G6-cystamine-(Gd-DO3A) resulted in significant contrast enhancement in the blood for about 5 mins, and Gd-DO3A was released from the conjugate and rapidly excreted via renal filtration after the disulfide spacer was cleaved. The nondegradable control had much longer blood circulation and excreted more slowly from the body. PAMAM-G6-cystamine-(Gd-DO3A) also resulted in more prominent tumor contrast enhancement than the control. However, PAMAM-G6-cystamine-(Gd-DO3A) demonstrated high toxicity due to the intrinsic toxicity of PAMAM dendrimers. In conclusion, although PAMAM-G6-cystamine-(Gd-DO3A) showed some advantages compared with the nondegradable control, PAMAM dendrimers are not suitable carriers for biodegradable macromolecular MRI contrast agents, due to their high toxicity.     

Synthesis and characterization of a theranostic vascular disrupting agent for in vivo MR imaging

Colchicine, a known tubulin binding agent and vascular disrupting agent, causes rapid vascular shut down and central necrosis in tumors. The binding of tubulin results in tubulin destabilization, with characteristic cell shape changes and inhibition of cell division, and results in cell death. A gadolinium(III) labeled derivative of colchicine (Gd·DOTA·Colchicinic acid) was synthesized and characterized as a theranostic agent (enabling simultaneous diagnostic/real time MRI contrast imaging). In vitro, Gd·DOTA·Colchicinic acid was shown to initiate cell changes characteristic of tubulin-destabilization in both OVCAR-3 and IGROV-1 ovarian carcinoma cell lines in vitro over a period of 24 h, while maintaining the qualities of the MR imaging tracer. In vivo, Gd·DOTA·Colchicinic acid (200 mg/kg) was shown to induce the formation of central necrosis, which was confirmed ex vivo by histology, in OVCAR-3 subcutaneous tumor xenografts, while simultaneously acting as an imaging agent to promote a significant reduction in the MR relaxation time T(1) (p < 0.05) of tumors 24 h post-administration. Morphological changes within the tumor which corresponded with areas derived from the formation of central necrosis were also present on MR images that were not observed for the same colchicine derivate that was not complexed with gadolinium that also presented with central necrosis ex vivo. However, Gd·DOTA·Colchicinic acid accumulation in the liver, as shown by changes in liver T(1) (p < 0.05), takes place within 2 h. The implication is that Gd·DOTA·Colchicinic acid distributes to tissues, including tumors, within 2 h, but enters tumor cells to lower T(1) times and promotes cell death over a period of up to 24 h. As the biodistribution/pharmacokinetic and pharmacodynamics data provided here is similar to that of conventional colchicines derivatives, such combined data are a potentially powerful way to rapidly characterize the complete behavior of drug candidates in vivo.     

PEG-g-poly(GdDTPA-co-L-cystine): a biodegradable macromolecular blood pool contrast agent for MR imaging

Biodegradable PEGylated Gd-DTPA l-cystine copolymers, PEG-g-poly(GdDTPA-co-l-cystine), were prepared and tested as a blood pool contrast agent in mice. The biodegradable macromolecular agent was designed to be broken down into smaller Gd complexes by endogenous thiols via the disulfide-thiol exchange reaction to facilitate the clearance of Gd complexes after the contrast-enhanced MRI examination. Gd-DTPA l-cystine copolymers were synthesized by condensation polymerization of l-cystine and DTPA-dianhydride in water followed by chelating with Gd(OAc)(3). MPEG-NH(2) (MW = 2000) was then conjugated to the polymeric backbone in different ratios. The macromolecular contrast agent was readily degraded with the incubation of l-cysteine. It also demonstrated superior contrast enhancement in the heart and blood vessels as compared to a low molecular weight control agent, Gd-(DTPA-BMA). At 1 h postcontrast, the PEGylated macromolecular agent still showed prominent enhancement, while little contrast enhancement was detectable in the blood pool by the control agent. PEG-g-poly(GdDTPA-co-l-cystine) shows promise as an MR blood pool imaging agent.     

Polydisulfide Gd(III) chelates as biodegradable macromolecular magnetic resonance imaging contrast agents

Macromolecular gadolinium (Gd)(III) complexes have a prolonged blood circulation time and can preferentially accumulate in solid tumors, depending on the tumor blood vessel hyperpermeability, resulting in superior contrast enhancement in magnetic resonance (MR) cardiovascular imaging and cancer imaging as shown in animal models. Unfortunately, safety concerns related to these agents' slow elimination from the body impede their clinical development. Polydisulfide Gd(III) complexes have been designed and developed as biodegradable macromolecular magnetic resonance imaging (MRI) contrast agents to facilitate the clearance of Gd(III) complexes from the body after MRI examinations. These novel agents can act as macromolecular contrast agents for in vivo imaging and excrete rapidly as low-molecular-weight agents. The rationale and recent development of the novel biodegradable contrast agents are reviewed here. Polydisulfide Gd(III) complexes have relatively long blood circulation time and gradually degrade into small Gd(III) complexes, which are rapidly excreted via renal filtration. These agents result in effective and prolonged in vivo contrast enhancement in the blood pool and tumor tissue in animal models, yet demonstrate minimal Gd(III) tissue retention as the clinically used low-molecular-weight agents. Structural modification of the agents can readily alter the contrast-enhancement kinetics. Polydisulfide Gd(III) complexes are promising for further clinical development as safe, effective, biodegradable macromolecular MRI contrast agents for cardiovascular and cancer imaging, and for evaluation of therapeutic response.