Maryke Kahts ^1* Beverley Summers ¹ Akhona N. Ndlela ² Aadil Gutta ^2,3 Phumudzo Nemutaduni ² Andrew More ² Aman Parsoo ² Thomas Ebenhan ^4,5 Jan R. Zeevaart ^4,5,6 Omer Aras ⁷ Mike Sathekge ⁴

• ¹ School of Pharmacy, Sefako Makgatho Health Sciences University, Pretoria, South Africa
• ² Dr George Mukhari Hospital, Ga-Rankuwa, South Africa
• ³ School of Medicine, Sefako Makgatho Health Sciences University, Pretoria, South Africa
• ⁴ Nuclear Medicine Research Infrastructure, Pretoria, South Africa
• ⁵ South African Nuclear Energy Corporation (South Africa), Pretoria, South Africa
• ⁶ PreClinical Drug Development Platform, North-West University, Potchefstroom, South Africa
• ⁷ Memorial Sloan Kettering Cancer Center, New York, New York, United States

Nuclear medicine infection imaging is routinely performed with the use of leukocytes radiolabelled with technetium-99m hexamethylpropyleneamine oxime ([99mTc]Tc-HMPAO) and single-photon emission computed tomography (SPECT). Positron emission tomography (PET) is more sensitive than SPECT and results in higher-quality images. Zirconium-89 (89Zr) is a positron emitter with a half-life of 78.4 hours, which translates to the biological half-life and slow biodistribution of intact cells, and allows delayed PET imaging for more accurate biodistribution of the labelled leukocytes to infection foci. A first-in-human study with [89Zr]Zr-oxine-leukocytes in four healthy volunteers was reported in 2022. Our first-in-human study utilising the cell surface labelling approach aimed to image infection in patients with the use of 89Zr-labelled leukocytes, using p-isothiocyanatobenzyl-desferrioxamine B (Df-Bz-NCS) as a bifunctional chelating agent, and to compare the scan quality and biodistribution of [89Zr]Zr-Df-Bz-NCS-labelled leukocytes on PET images to SPECT images obtained with [99mTc]Tc-HMPAO-labelled leukocytes. Leukocytes were isolated from whole-blood samples of eight patients with clinically and/or radiologically confirmed infection. Isolated leukocytes were labelled with [99mTc]Tc-HMPAO according to standardised methods, and [89Zr]Zr-Df-Bz-NCS according to our previously published radiolabelling method. Whole-body SPECT imaging was performed 2 h and 18 h post-injection of [99mTc]Tc-HMPAO-labelled leukocytes, and whole-body PET/CT was performed 3 h and 24 h post-injection of [89Zr]Zr-Df-Bz-NCS-labelled leukocytes in 7 patients. Successful [89Zr]Zr-Df-Bz-NCS-leukocyte labelling was achieved. High labelling efficiencies were obtained (81.7 ± 3.6%; n=8). An average high viability of [89Zr]Zr-Df-Bz-NCS-labelled leukocytes was observed (88.98 ± 12.51%). The [89Zr]Zr-Df-Bz-NCS-leukocyte labelling efficiency was not significantly affected by the white blood cell count of the patient. The performance of [99mTc]Tc-HMPAO- and [89Zr]Zr-Df-Bz-NCS-labelled leukocytes, in terms of the ability to accurately detect infection, were similar in two out of seven patients, and [99mTc]Tc-HMPAO-labelled leukocytes outperformed [89Zr]Zr-Df-Bz-NCS-labelled leukocytes in one patient with femoral osteomyelitis. However, in two cases of pulmonary pathology, [89Zr]Zr-Df-Bz-NCS-labelled leukocytes demonstrated improved pathological uptake. No skeletal activity was observed in any of the patients imaged with [89Zr]Zr-Df-Bz-NCS-labelled leukocytes, illustrating the in vivo stability of the radiolabel. Although the [89Zr]Zr-Df-Bz-NCS-leukocyte labelling aspect of this study was noteworthy, infection imaging did not yield convincingly positive results due to the pulmonary trapping of intravenously administered [89Zr]Zr-Df-Bz-NCS-labelled leukocytes.