90 Y/ 177 Lu-labelled Cetuximab immunoconjugates: radiochemistry optimization to clinical dose formulation Rubel Chakravarty, a * Sudipta Chakraborty, a Haladhar Dev Sarma, b K. V. Vimalnath Nair, a Ardhi Rajeswari a and Ashutosh Dash a * Radiolabelled monoclonal antibodies (mAbs) are increasingly being utilized in cancer theranostics, which is a significant move toward tailored treatment for individual patients. Cetuximab is a recombinant, human–mouse chimeric IgG 1 mAb that binds to the epidermal growth factor receptor with high affinity. We have optimized a protocol for formulation of clinically relevant doses (~2.22 GBq) of 90 Y-labelled Cetuximab and 177 Lu-labelled Cetuximab by conjugation of the mAb with a suitable bifunctional chelator, N-[(R)-2-amino-3-(paraisothiocyanato-phenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diamine-N, N,N′,N″,N″-pentaacetic acid (CHX-A″-DTPA). The radioimmunoconjugates demonstrated reasonably high specific activity (1.26 ± 0.27 GBq/mg for 90 Y-CHX-A″-DTPA-Cetuximab and 1.14 ± 0.15 GBq/mg for 177 Lu-CHX-A″-DTPA-Cetuximab), high radiochemical purity (>95%) and appreciable in vitro stability under physiological conditions. Preliminary biodistribution studies with both 90 Y-CHX-A″-DTPA-Cetuximab and 177 Lu-CHX-A″-DTPA-Cetuximab in Swiss mice bearing fibrosarcoma tumours demonstrated significant tumour uptake at 24-h post-injection (p.i.) (~16%ID/g) with good tumour-to-background contrast. The results of the biodistribution studies were further corroborated by ex vivo Cerenkov luminescence imaging after administration of 90 Y-CHX-A″-DTPA-Cetuximab in tumour-bearing mice. The tumour uptake at 24 h p.i. was significantly reduced with excess unlabelled Cetuximab, suggesting that the uptake was receptor mediated. The results of this study hold promise, and this strategy should be further explored for clinical translation. Keywords: cancer; cetuximab; EGFR; lutetium-177; theranostics; yttrium-90 Introduction Over the last several years, there is an increasing thrust towards use of radiolabelled monoclonal antibodies (mAbs) with high affinity towards tumour-associated antigens for cancer-specific imaging and therapy. 1,2 One of the most promising targets for this purpose is the family of epidermal growth factor receptors (EGFRs). 2,3 Overexpression of EGFR is the characteristic of many human tumours, such as head and neck squamous cell carcinoma (HNSCC), gastrointestinal and abdominal carcinomas, lung carcinomas, soft tissue carcinomas, breast carcinomas, carcinomas of the reproductive tract, melanomas, glioblastomas and thyroid carcinomas. 2,3 In several cases, EGFR overexpression may act as a prognostic indicator, predicting poor survival and/or more advanced disease stage. Cetuximab (IMC-C225), an EGFR antagonist, is a chimeric human-mouse mAb commercially available as Erbitux® (Merck KGaA, Darmstadt, Germany). 2 This mAb has been approved by the United States Food and Drug Administration for treatment of colorectal cancer or HNSCC. Over the last decade, there have been numerous reports on preparation of radiolabelled Cetuximab conjugates for cancer management. 2,4–7 Among various radionuclides studied, 90 Y (t ½ = 64.1 h) and 177 Lu (t ½ = 6.7 days) are the most frequently used radionuclides in radioimmunotherapy (RIT) of tumours. 8–10 Yttrium-90 is a high- energy β À emitter (E max = 2.28 MeV) that gives higher dose rate because of its shorter half-life. Its long emission range can penetrate to tissues farther away from the target tissue and kill the adjoining cancerous tumour cells that may not have bound the radiolabelled Cetuximab. This radioisotope is especially advantageous in larger tumours and in tumours with heterogeneous receptor distribution. On the other hand, 177 Lu has lower energy and smaller particle range [emits β À particles with E max of 497 keV (78.6%), 384 keV (9.1%) and 176 keV (12.2%)], leading to better absorption in smaller tumours. Additionally, this radioisotope emits 208 keV γ-radiation which is suitable for single photon emission computed tomography (SPECT) imaging and also enables dosimetry during RIT. In order to enable radiolabelling with a metallic radioisotope such as 90 Y or 177 Lu, Cetuximab must be conjugated with a suitable bifunctional chelator (BFC). The selection of the BFC can significantly affect both the radiochemical and biological a Isotope Production and Applications Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India b Radiation Biology and Health Sciences Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India *Correspondence to: Ashutosh Dash and Rubel Chakravarty, Isotope Production and Applications Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India. E-mail: adash@barc.gov.in; rubelc@barc.gov.in Copyright © 2016 John Wiley & Sons, Ltd. J. Label Compd. Radiopharm 2016, 59 354–363 Research article Received 09 March 2016, Revised 05 May 2016, Accepted 10 May 2016 Published online 5 June 2016 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/jlcr.3413 354