With available MRI techniques, primary and metastatic liver cancers that are associated with high mortality rates and poor treatment responses are only diagnosed at late stages, due to the lack of highly sensitive contrast agents without Gd 3+ toxicity. We have developed a protein contrast agent (ProCA32) that exhibits high stability for Gd 3+ and a 10 11 -fold greater selectivity for Gd 3+ over Zn 2+ compared with existing contrast agents. ProCA32, modified from parvalbumin, possesses high relaxivities (r 1 /r 2 : 66.8 mmol −1 ·s −1 / 89.2 mmol −1 ·s −1 per particle). Using T 1 -and T 2 -weighted, as well as T 2 /T 1 ratio imaging, we have achieved, for the first time (to our knowledge), robust MRI detection of early liver metastases as small as ∼0.24 mm in diameter, much smaller than the current detection limit of 10-20 mm. Furthermore, ProCA32 exhibits appropriate in vivo preference for liver sinusoidal spaces and pharmacokinetics for high-quality imaging. ProCA32 will be invaluable for noninvasive early detection of primary and metastatic liver cancers as well as for monitoring treatment and guiding therapeutic interventions, including drug delivery.MRI | uveal melanoma | metastasis | contrast agents | T 2 /T 1 ratio imaging T umor metastasis is the main cause of nearly all human cancerrelated deaths. Liver is a common site for metastases of a variety of cancers, including melanoma, breast, pancreatic, and colon cancers (1, 2). For example, uveal melanoma, the most common primary intraocular tumor, has a 40% risk of metastasizing to the liver within 10 y of diagnosis of the primary tumor. Hepatic metastases, which occur in 95% of patients with uveal melanoma metastasis, result in death in almost all cases. This high death rate is related to the recognition of liver metastasis at a late clinical stage (at stage II or later in the TNM system) in which the metastatic uveal melanoma is resistant to currently available systemic chemotherapies (3, 4). The liver may also give rise to primary tumors such as hepatocellular carcinoma (HCC), which is the most common primary malignancy worldwide (5). However, currently there is no reliable way to detect primary liver cancer and hepatic metastases at early stages with high sensitivity and specificity.MRI is a widely used clinical imaging modality that provides exquisite soft-tissue contrast without using ionizing radiation (6, 7). More than 35% of MRI scans use MRI contrast agents, particularly paramagnetic gadolinium (Gd 3+ )-based contrast agents, which shorten T 1 and lead to an increase in MRI intensity (8). All clinically approved Gd 3+ -containing contrast agents are based on small chelators with relaxivity (r 1 and r 2 ) values around 4-6 mM −1 ·s −1 . Their low relaxivities severely limit the sensitivity of current MR imaging methods with respect to detection of lesions and treatment planning and monitoring. Repeat MRI scans are frequently requested for patients with ambiguous small lesions and are used as a routine follow-up modality for highrisk patients, but t...

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Prostate-specific membrane antigen (PSMA) is one of the most specific cell surface markers for prostate cancer diagnosis and targeted treatment. However, achieving molecular imaging using non-invasive MRI with high resolution has yet to be achieved due to the lack of contrast agents with significantly improved relaxivity for sensitivity, targeting capabilities and metal selectivity. We have previously reported our creation of a novel class of protein Gd 3+ contrast agents, ProCA32, which displayed significantly improved relaxivity while exhibiting strong Gd 3+ binding selectivity over physiological metal ions. In this study, we report our effort in further developing biomarker-targeted protein MRI contrast agents for molecular imaging of PSMA. Among three PSMA targeted contrast agents engineered with addition of different molecular recognition sequences, ProCA32.PSMA exhibits a binding affinity of 1.1 ± 0.1 μM for PSMA while the metal binding affinity is maintained at 0.9 ± 0.1 × 10 −22 M. In addition, ProCA32.PSMA exhibits r 1 of 27.6 mM −1 s −1 and r 2 of 37.9 mM −1 s −1 per Gd (55.2 and 75.8 mM −1 s −1 per molecule r 1 and r 2 , respectively) at 1.4 T. At 7 T, ProCA32.PSMA also has r 2 of 94.0 mM −1 s −1 per Gd (188.0 mM −1 s −1 per molecule) and r 1 of 18.6 mM −1 s −1 per Gd (37.2 mM −1 s −1 per molecule). This contrast capability enables the first MRI enhancement dependent on PSMA expression levels in tumor bearing mice using both T 1 and T 2 -weighted MRI at 7 T. Further development of these PSMAtargeted contrast agents are expected to be used for the precision imaging of prostate cancer at an † Electronic supplementary information (ESI) available. See

Magnetic resonance imaging (MRI) of disease biomarkers, especially cancer biomarkers, could potentially improve our understanding of the disease and drug activity during preclinical and clinical drug treatment and patient stratification. MRI contrast agents with high relaxivity and targeting capability to tumor biomarkers are highly required. Extensive work has been done to develop MRI contrast agents. However, only a few limited literatures report that protein residues can function as ligands to bind Gd 3+ with high binding affinity, selectivity, and relaxivity. In this paper, we focus on reporting our current progress on designing a novel class of protein-based Gd 3+ MRI contrast agents (ProCAs) equipped with several desirable capabilities for in vivo application of MRI of tumor biomarkers. We will first discuss our strategy for improving the relaxivity by a novel protein-based design. We then discuss the effect of increased relaxivity of ProCAs on improving the detection limits for MRI contrast agent, especially for in vivo application. We will further report our efforts to improve in vivo imaging capability and our achievement in molecular imaging of cancer biomarkers with potential preclinical and clinical applications.