An engineered cyclic peptide provides structural constraints to resist non-specific degradation in the human body and includes environment-specific cleavage sites to allow release of a linearized peptide upon reaching a target environment. The linearized peptide can include a reporter molecule or a bioactive therapeutic such that the cyclic peptide is essentially inactive at administration and in circulation but becomes reactive only upon exposure to target-specific environmental factors such as a specific combination of differentially-expressed proteases associated with a target tissue or disease state. The peptides can include tuning that modulate distribution by targeting the particle to specific tissue, bodily fluids, or cell types.
