96 Chapter 5 Altogether, PET-imaging with 89Zr-labeled PD-L1 antibodies has not consistently shown a correlation between tracer-uptake and treatment response. Potential explanations are the different characteristics of the antibodies used, which include 1) affinity for PD-L1 which could influence tumor retention; 2) Fc-tail modification/glycosylation which could affect circulation time and effector functions; 3) non-specific antibody-uptake due to enhanced permeability and retention (EPR) effect 34-36. As a result of the EPR-effect, there is always a (low) PET-signal in the tumor, while this is not PD-L1 mediated. This may hamper the detection of small amounts of tumor PD-L1, which can be clinically relevant as low PD-L1 expression (1% positive cells) has been associated with ICI response. To limit the non-specific-uptake and thereby increase the potential to measure low PD-L1 expression levels, small molecules or peptides with rapid blood clearance can be used 37,38. Finally, other mechanisms within immune suppressive microenvironment beyond PD-L1, such as the activation and promoting of CD8+ T-cell priming in tumor draining lymph nodes determine ICI response could have influenced the correlation between tracer-uptake and ICI response39. Despite the fact that [89Zr]Zr-DFO-durvalumab did not correlate to treatment outcome, we do see potential of 89Zr-labeled antibodies in optimizing the ICI treatment efficacy in patients with R/M SCCHN 40. Besides an unique insight in antibody biodistribution, the in vivo visualization of 89Zr-labeled antibodies highlights essential local effector mechanisms, reveals the complexity of dose-response relations, and may shed a new light on the role of non-tumor located PD-L1 expression in the anti-cancer immune responses 39. Ultimately, this teaches us how to use (and combine) these drugs to improve response rates; an essential step in early drug development suitable for phase 1 and 2 clinical trials.