Table 2.

Challenges and possible solutions for the development and testing of novel antibiotic‐based radiopharmaceuticals for infection imaging.

Challenge		Possible strategy/solution		Limitation
Antibiotic radiolabeling protocol is unable to match its antibiotic MoA		‐ libraries & SAR (target binding efficacy) ‐ computational tests (aim at preserving the pharmacophore)		‐ radioisotope production and radiopharmaceutical aspects (low specific activity)
Risk of compromised tracer sensitivity		‐ select antibiotics that target highly active/expressed biological processes ‐ disregard antibiotics with MoA that are not well understood ‐ consider the mass effect of tracer formulation ‐ radiosynthesis optimization (formulation, dosage, carrier content); following quality guidelines ‐ testing tracer sensitivity in non‐human primate models or first‐in human investigations prior to clinical trials		‐ biological target expression is underwhelming ‐ threshold B max [a]/K d [b] may decrease <10 for antibiotics derivatives ‐ radiotracer: inadequate specific activity ‐ small animal models only acceptable for proof‐of‐principle investigation
Risk associated with accuracy of visualizing infection		‐ disregard antibiotics with predisposed MoA ‐ drug resistant pathogens: use vectors that circumvent/take advantage of defense mechanism, i.e., target overexpression or genetic redundancy		‐ presence of additional (cold) ligand or conflicting pathogen environment ‐ cumbersome prodrug activation processes ‐ pre‐treated subjects using widely prescribed antibiotics
Effects of empiric use of antibiotics		‐ opting for radiosynthesis of antibiotics with unique MoA is crucial		‐ radiotracer: inadequate specific activity
Unwanted (altered) tracer bioavailability and biodistribution		‐ ADME: prioritize antibiotics with rapid clearance from high‐risk organ/compartments for infection ‐ assess candidates for host enzymatic and tissue specific interactions ‐ practice SAR‐guided incorporation of a radiolabeled functional group ‐consideration of liposome‐, nanoparticle, or microsphere‐based delivery system (transfer intact tracer to target) ‐ permit radionuclide incorporation only to non‐cleavable structures		‐ relatively long biological half‐life of antibiotics ‐ antibiotics sometimes associate with host inflammatory response ‐ unforeseen shifts in physico‐chemical properties (lipophilicity by carbon chain spacers/polarity by metal chelator conjugation) can occur

[a] target protein/receptor density; [b] binding disassociation constant.