Abstract. Experience with imaging of the multi-drug resistance (MDR) phenotype in tumours using techne- tium-99m sestamibi, a substrate of the P-glycoprotein (Pgp) transporter, suggests that better quantification of images and separation of MDR from other variables affecting tracer uptake in tumours are required. One approach to these problems is the development of short half-life positron-emitting tracers which are substrates of Pgp. Several lipophilic cationic copper(I) bis(diphos- phine) complexes labelled with copper-64 have been synthesised and evaluated in vitro as substrates for Pgp. The synthesis is rapid and efficient with no need for purification steps. The chemistry is suitable for use with very short half-life radionuclides such as copper-62 (9.7 min) and copper-60 (23.7 min). Incubation of the complexes with human serum in vitro showed that they are sufficiently stable in serum to support clinical imag- ing, and the more lipophilic members of the series are taken up rapidly by cells (Chinese hamster ovary and hu- man ovarian carcinoma) in vitro with great avidity. Up- take in human ovarian carcinoma cells is significantly reduced after several months of conditioning in the pres- ence of doxorubicin, which induces increased Pgp ex- pression. Uptake in hooded rat sarcoma (HSN) cells, which express Pgp, is significantly increased in the pres- ence of the MDR modulator cyclosporin A. Biodistribu- tion studies in hooded rats show rapid blood clearance, excretion through both kidneys and liver, and low uptake in other tissues. The one complex investigated in HSN tumour-bearing rats showed uptake in tumour increasing up to 30 min p.i. while it was decreasing in other tissues. We conclude that diphosphine ligands offer a good basis for development of radiopharmaceuticals containing copper radionuclides, and that this series of compl should undergo further evaluation in vivo as positr emission tomography imaging agents for MDR. Key words: Copper-64 – Multi-drug resistance – Diphos phines – Positron emission tomography – Tumour Eur J Nucl Med (2000) 27:638–646 Introduction The use of radionuclide imaging methods to detect multi-drug resistance (MDR) phenotype [1, 2] in tu mours has attracted growing interest in recent years [3 and has the potential to inform decisions on manageme of individual cancer patients [4]. MDR can arise throug several mechanisms, the best understood of which volve one or more membrane transport systems that ac tively pump anti-tumour drugs from cells. The earliest o these to be identified was the mdr-1 gene product P-gly coprotein, Pgp (P-170). Other efflux pumps have si been described: multi-drug resistance associated prote (MRP-1) [5] and its more recently discovered homo logues MRP-2 to -6 [6]. MRP-1 has a similar “portfolio” of drug substrates to Pgp. Despite incomplete unde standing of the transport mechanisms, the observat that lipophilic cations are a generic class of molec able to act as substrates for Pgp has led to the identific tion and assessment of technetium-99m sestamibi [ and other 99m Tc-containing lipophilic cations (e.g. 99m Tc- tetrofosmin [8] and 99m Tc-furifosmin [9] and analogues [10]) as prototype imaging agents for imaging of Pgp ex pression in vivo [11]. Uptake of this tracer in tumours i often higher than in surrounding normal tissue [12], pr sumably as a result of abnormal blood flow and m chondrial volume or activity [13]. However, uptake is re duced if the tumour cells are multi-drug resistant [11, 1 Present address: J.S. Lewis, Mallinckrodt Institute of Radiology, Washington University, School of Medicine, St Louis, MO63110, USA Correspondence to: Philip J Blower, Nuclear Medicine Depart- ment, Kent and Canterbury Hospital, Canterbury CT1 3NG, UK, e-mail: P.J.Blower@ukc.ac.uk, Tel.: +44-1227-766877 ext 4774, Fax: +44-1227-783059 Original article Copper bis(diphosphine) complexes: radiopharmaceuticals for the detection of multi-drug resistance in tumours by PE Jason S. Lewis 1 , Jason L.J. Dearling 1 , Jane K. Sosabowski 2 , Jamal Zweit 2 , Paul Carnochan 2 , Lloyd R. Kelland 2 , Helen M. Coley 2 , Philip J. Blower 1 1 Research School of Biosciences, University of Kent, Canterbury CT2 7NJ, UK 2 Joint Department of Physics and CRC Centre for Cancer Therapeutics, Institute for Cancer Research, Royal Marsden Hospital, Sutton SM2 5PT, UK Received 14 December 1999 and in revised form 12 February 2000 European Journal of Nuclear Medicine Vol. 27, No. 6, June 2000 – © Springer-Verlag 2000