PET imaging with copper‐64 as a tool for real‐time in vivo investigations of the necessity for cross‐linking of polymeric micelles in nanomedicine

Polymeric micelles in nanomedicine are often cross‐linked to prevent disintegration in vivo. This typically requires clinically problematic chemicals or laborious procedures. In addition, cross‐linking may interfere with advanced release strategies. Despite this, it is often not investigated whether cross‐linking is necessary for efficient drug delivery. We used positron emission tomography (PET) imaging with 64Cu to demonstrate general methodology for real‐time in vivo investigations of micelle stability. Triblock copolymers with 4‐methylcoumarin cores of ABC‐type (PEG‐PHEMA‐PCMA) were functionalized in the handle region (PHEMA) with CB‐TE2A chelators. Polymeric micelles were formed by dialysis and one half was core cross‐linked (CL) by UV light and the other half was not (nonCL). Both CL and nonCL were radiolabeled with 64Cu and compared in vivo in tumor‐bearing mice, with free 64Cu as control. Accumulation in relevant organs was quantified by region of interest analysis on PET images and ex vivo counting. It was observed that CL and nonCL showed limited differences in biodistribution from each other, whereas both differed markedly from control (free 64Cu). This demonstrated that 4‐methylcoumarin core micelles may form micelles that are stable in circulation even without cross‐linking. The methodology presented here where individual unimers are radiolabeled is applicable to a wide range of polymeric micelle types. Polymeric micelles for drug delivery are often cross‐linked, but good methods for investigating the effects of cross‐linking in vivo are lacking. We radiolabeled single coumarin‐containing micelle unimers with 64Cu through CB‐TE2A and compared cross‐linked with noncross‐linked micelles by in vivo PET imaging, with free 64Cu as control. As both micelle types showed similar biodistribution, we could conclude that coumarin‐core micelles may not require cross‐linking, as well as demonstrate general methodology for in vivo investigation of cross‐linking.