Biodistribution of canine leucocytes labelled with technetium-99m stannous fluoride colloid in whole blood and their ability to localise to sites of induced inflammation

Abstract

This study assessed the biodistribution of autologous leucocytes radiolabelled with technetium-99m stannous fluoride colloid (^99mTcSnC) for detection of foci of induced inflammation in dogs. Venous blood was collected from seven healthy dogs and incubated with ^99mTcSnC for 1 h at room temperature. Radiolabelled samples were injected intravenously (IV) and the dogs were scanned using a gamma camera. Another seven healthy dogs were injected intradermally with tumour necrosis factor α and then IV with ^99mTcSnC radiolabelled autologous blood 3 h later before being scanned. The radiolabelled leucocytes localised to sites of inflammation by 30 min post-injection. IV injection of autologous leucocytes radiolabelled with ^99mTcSnC appears to be a sensitive method for localisation of induced foci of inflammation in dogs.

Introduction

Radiolabelled autologous leucocytes are a tool used in nuclear medicine to detect a range of infectious and non-infectious inflammatory diseases in humans (Chianelli et al., 1997, Van der Laken et al., 1998). Leucocytes can be labelled indirectly (in vivo) or directly (in vitro); in vivo techniques include radiolabelling anti-granulocyte monoclonal antibodies (Becker et al., 1994, Becker et al., 1996), chemotactic peptides (Fischman et al., 1991) and cytokines, such as interleukin (IL)-1 (Van der Laken et al., 1995), IL-8 (Bleeker-Rovers et al., 2007) and platelet factor-4 (Moyer et al., 1996).

In vitro labelling of leucocytes has been performed using three different methods. The first is a stannous reduction technique in which isolated leucocytes are treated with stannous citrate prior to incubation with technetium-99m (^99mTc) sodium pertechnetate (Uchida and Vincent, 1976, Kelbaek and Fogh, 1985). The second method, most commonly used in human medicine, uses a lipophilic complex that passively penetrates the leucocyte cell membrane and attaches to intracellular receptors. Such agents include indium-111-oxine (¹¹¹In-oxine) and ^99mTc-hexamethylpropylene amine oxime (^99mTc-HMPAO). However, the technique is labour-intensive, time-consuming and expensive (Chianelli et al., 1997, Van der Laken et al., 1998).

The third in vitro labelling method involves leucocyte uptake of hydrophilic compounds such as ^99mTc stannous fluoride colloid (^99mTcSnC) by phagocytosis (Hanna et al., 1984, Hanna and Lomas, 1986, Gibson et al., 1991, Puncher and Blower, 1995, Ramsay et al., 2001; Gallagher et al., 2006). Human leucocytes can be labelled with ^99mTcSnC either in whole blood or in leucocyte-rich plasma (LRP). Greater than 95% radiolabelling efficiency has been achieved in whole blood (Hanna and Lomas, 1986, Pullman et al., 1988, Gibson et al., 1991, Boyd et al., 1993, Carter et al., 2002, Peacock et al., 2004). In LRP, the reported radiolabelling efficiency was 75% in one study (Puncher and Blower, 1995) and 45% in another (Mock and English, 1987).

^99mTcSnC has some advantages over other in vitro labelling agents for human leucocytes. It can be added to whole blood without the need to separate leucocytes (Hanna et al., 1984, Boyd et al., 1993, Carter et al., 2002) and it is less expensive and more readily available than other radiolabelling compounds used for this purpose. Other important clinical advantages of ^99mTcSnC-labelled leucocytes in humans include low bowel uptake in early images (4 h post-injection) and delayed faecal excretion when compared with ^99mTc-HMPAO-labelled leucocytes. Therefore, ^99mTcSnC-labelled leucocytes are useful in the assessment of human patients with suspected inflammatory bowel disease (IBD) (Pullman et al., 1988, Gibson et al., 1991, Peacock et al., 2004).

Leucocytes labelled with ^99mTcSnC have been used successfully in Australia for more than 15 years to diagnose a variety of inflammatory disorders in humans (Tsopelas, 2005). Injection of autologous leucocytes labelled in whole blood with ^99mTcSnC is comparable in sensitivity and specificity to the injection of ¹¹¹In-oxine-labelled leucocytes in the diagnosis of active IBD, musculoskeletal sepsis, acute osteomyelitis and prosthetic sepsis, and in identifying the sources of post-operative pyrexia and pyrexia of unknown origin (Boyd et al., 1993, Carter et al., 2002).

An in vivo study in a single human volunteer demonstrated that a large proportion of ^99mTcSnC-labelled leucocytes accumulated in the lungs, liver and spleen at 30 min post-injection (Mock and English, 1987). Pulmonary uptake ceased by 3–4 h post-injection, whereas hepatic and splenic uptake persisted until 6 h post-injection. A comparison between ^99mTcSnC-labelled leucocytes and three-phase bone scanning for inflammatory arthropathies demonstrated that ^99mTcSnC-labelled leucocytes are less sensitive but more specific than three-phase bone scanning (Chick et al., 1996).

Canine leucocytes have been radiolabelled with ^99mTcSnC in whole blood (Abushhiwa et al., 2006, Abushhiwa et al., 2008) and in LRP (Hirsch et al., 1989), with reported labelling efficiencies of 98.9 ± 0.1% and 89.5 ± 4.3%, respectively. Radiolabelling of canine leucocytes with ^99mTc-SnC does not adversely affect their viability or phagocytic function (Hirsch et al., 1989, Abushhiwa et al., 2006, Abushhiwa et al., 2008). The present study investigated the biodistribution of canine leucocytes radiolabelled with ^99mTcSnC in whole blood and the ability of such cells to localise to sites of induced inflammation in dogs.