Table 1.

Pros and cons of imaging technologies

	Target	Tracer	Application	Pros	Cons
T cell-specific marker	murine TCR	89Zr-DFO-aTCRmu-F(ab′)2, 64Cu-cOVA-TCR	TILs	• fast and robust accumulation of the probe in the cells due to high rate of internalization of the TCR • reliable correlation between cell number and the signal	• tracer may induce T cell stimulation • less signal may be seen after cell division
	murine CD8	89Zr-malDFO-169 cDb	TILs	• decreased aggregation, more stability in blood, and more successful lymph node and spleen targeting	• antibody clearance is faster than full antibodies
	human CD8	89Zr-DFO-IAB22M2C
	murine CD3	89Zr-DFO-CD3	TILs	• provides visualization of all T cell populations • eliminates need for specific pathway, as CD3 is a pan-T cell marker • correlated with immune response	• requires extensive testing and optimization before being translated into the clinic
T cell activation marker	CD25	99mTc-HYNIC-IL-2	TILs	• CD25 is highly expressed in activated T cells	• patients may experience adverse events because radiolabeled IL-2 is biologically active
	OX40	64Cu-DOTA-AbOX40	TILs	• not only monitors T cells but also shows responder T cells • OX40 represents an early activation marker and therefore provides quick assessment	• OX40 expression on T cells is diverse, and thus deviation between tracer uptake and cell number may be seen
Metabolic pathway	deoxycytidine kinase (dCK)	18F-F-AraC, 18F-CFA	TILs	• activated T cells increase the entry of the substrate	• tumors also uptake the tracer
	deoxyguanosine kinase (dGK)	18F-F-AraG	TILs	• activated T cells increase the entry of the substrate	• high tracer uptake in the background
T cell effector molecules	human PD-1	64Cu-DOTA-anti-PD-1, 89Zr-nivolumab, 89Zr-penbrolizumab	TILs	• correlation between biodistribution of tracer and IHC	• requires several days for the tracer to accumulate to the target
	human CTLA-4	64Cu-DOTA-anti-CTLA-4, 89Zr-ipilimumab	TILs	• correlation between biodistribution of tracer and IHC	• clinical trial is ongoing
	murine granzyme B	68Ga-NOTA-GZP	TILs	• correlation between biodistribution of tracer and treatment response	• low dependence on tissue migration • no clinical trials to date
	murine IFN-γ	89Zr-anti-IFN-γ	TILs	• correlation between biodistribution of tracer and treatment response	• low dependence on tissue migration • no clinical trials to date
Reporter gene	HSV-tk	18F-FHBG	oncolytic virus, CART cells	• early clinical trial showed safety and feasibility of imaging CART cells with HSV-tk	• tracers do not penetrate BBB • immunogenicity
	NIS	124I, 18F-TFB, 18F-SO3F	oncolytic virus, CART cells, regulatory T cells, dendritic cells	• no immune response • high sensitivity • no toxicity to transduced cells	• tracers do not penetrate BBB • endogenous expression is seen in thyroid, stomach, and salivary gland
	SSTr2	68Ga-DOTANOC, 68Ga-DOTATOC, 68Ga-DOTATATE	CART cells, TILs	• no immune response • high sensitivity • no toxicity to transduced cells	• endogenous expression in several organs and cancer types • tracers may activate T cells
	D2R	11C-raclopride, 18F-FESP	–	• tracer penetrates BBB	• has not been reported for tracking T cells in vivo
	hdCK	124I-FIAU, 18F-FEAU	CART cells, TILs	• non-immunogenic	• endogenous expression in several organs and cancer types • tracers do not penetrate BBB • accumulates less pyrimidine-based radiotracer compared to HSV-tk
	eDHFR	18F-TMP	CART cells	• high sensitivity • tracer penetrates BBB	• immunogenicity
	PSMA	18F-DCFPyL, 18F-DCFBC, 68Ga-PSMA-11	CART cells	• new tracers that are cleared rapidly from kidney are under development • signal amplification due to receptor-tracer complex internalization	• kidney and patients with prostate cancer have background issue • possible issues with overexpression of PSMA

TCR, T cell receptor; PD-1, programmed cell death protein 1; CTLA-4, cytotoxic T-lymphocyte antigen 4; IFN, interferon; HSV-tk, herpes simplex virus thymidine kinase; NIS, sodium/iodide symporter; SSTr2, somatostatin receptor 2; D2R, dopamine 2 receptor; hdCK, human deoxycytidine kinase; eDHFR, Escherichia coli dihydrofolate reductase; PSMA, prostate-specific membrane antigen; DFO, desferrioxamine; muTCR, murine T cell receptor; F(Ab), antigen-binding fragment; cOVA, chicken ovalbumin; mal, malemide; cDb, cys-diabody; HYNIC, 6,-hydrazinonicotinamide; AraC, arabinofuranosyl cytidine; CFA, clofarabine; AraG, arabinofuranosyl guanine; GZP, granzyme B specific PET imaging agent; FHBG, fluoro-3-hydroxymethyl-butyl guanine; TFB, tetrafluoroborate; FESP, fluoroethyl spiperone; FIAU, 2′-fluoro-2′-deoxy-1β-d-arabinofuranosyl-5-iodouracil; FEAU, fluoro-5-ethyl-1-β-D-arabinofuranosyluracil; TMP, trimethoprim; ¹⁸F-DCFPyL, 2-(3-{1-carboxy-5-[(6-¹⁸F-fluoro-pyridine-3-carbonyl)-amino]-pentyl}-ureido)-pentanedioic acid; ¹⁸F-DCFBC, N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-4-¹⁸F-fluorobenzyl-L-cysteine; TILs, tumor infiltrating lymphocytes; CART, chimeric antigen receptor T cell; IHC, immunohistochemistry; BBB, blood–brain barrier