###### Abstract  
A novel, highly biocompatible, well soluble melanin-based probe obtained from the conjugation of melanin macromolecule to bovine serum albumin (BSA) was tested as a contrast agent for photoacoustic tumor imaging. Five soluble conjugates (PheoBSA A-E) were synthesized by oxidation of dopamine (DA) in the presence of variable amounts of BSA. All systems showed the similar size and absorbance spectra, being PheoBSA D (DA:BSA ratio 1:2) the one showing the highest photoacoustic efficiency. This system was then selected for the investigations as it showed a marked enhancement of the photoacoustic (PA) contrast in the tumor region upon iv injection. Biodistribution studies confirmed the accumulation of PheoBSA D within the tumor region and showed fast renal elimination, lack of cell toxicity, and good hemocompatibility. A higher PA contrast enhancement was observed in the case of PC3 prostate tumor xenograft when compared to the TS/A breast one, likely reflecting different vascularization/extravasation properties between the two tumor murine models. The improved PA properties shown by PheoBSA D allowed to set up a 3D dynamic contrast-enhanced (DCE) approach that demonstrated a persistent increase of the PA signal in the tumor region for a long period. Overall, the herein reported results demonstrate that PheoBSA D is a promising blood pool contrast agent for in vivo PA imaging, particularly useful for the set-up of 3D DCE-PA approaches to monitor tumor vascular properties. n
a 2020 10 8313 2  
Melanin signal has been exploited to assess the metastatic status of the sentinel lymph node in melanoma patients [8]. The contrast enhancement shown by endogenous molecules has inspired the design of exogenous PAI contrast agents (CAs) [9; 10]. The PAI-CAs allow to get improved contrast between anatomical structures and provide specific physiological information [11; 12; 13]. To date, exogenous PAI contrast agents are based on several platforms, including small-molecule dyes like indocyanine green [14; 15; 16], eventually confined in silica nanoparticles [17; 18], carbon nanoparticles [19; 20; 21], gold nanostructures [22; 23; 24; 25; 26], iron-based nanosystems [27], pyrroles [28; 29] and biodegradable particles [30]. All these newly developed CAs have been exploited for assessing several properties of the cancer microenvironment [31; 32; 33; 34; 35]. As for other medically oriented applications of exogenous nanoparticles, biocompatibility and toxicity issues may hamper the use of nano-sized PAI CAs towards their clinical translation. In fact, although they can be highly specific and provide good contrast, they often show slow renal clearance and may accumulate non-specifically in the mononuclear phagocyte system [36]. Accordingly, the design of less toxic nanoparticles based on endogenous molecules or optically active biodegradable materials attracted much attention in the last few years [10; 30]. Melanin is a ubiquitous natural biopolymer found in skin, hair, eyes, and other tissues and it was early recognized as a source of optoacoustic imaging response since it displays a marked absorption over a large spectral region [8; 37; 38; 39]. Melanins can be classified into four major categories [40] based on involved precursors, their color and distribution, namely (i) black to brown eumelanin derived from S-3,4-dihydroxyphenylalanine (L-dopa) consisting of oligomers of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) as main constituents; (ii) reddish to light brownish pheomelanin containing benzothiaze and benzothiazole units resulting from the oxidative polymerization of 5-S-cysteineyl-dopa or 2-S-cysteineyl-dopa[41]; (iii) allomelanin (mainly present in fungi and other microorganisms) obtained from the oxidation of catechols and 1,8-dihydroxynaphtalene; (iv) neuromelanin (NM), a complex macromolecular pigment found primarily in dopaminergic neurons of the human substantia nigra (SN) of the central nervous system.