An evaluation of six kits of technetium 99m human serum albumin injection for cardiac blood pool imaging.

We have investigated the suitability of five different commercially available kits which provide human serum albumin (HSA) labelled with technetium 99m (99mTc) for cardiac blood pool imaging. Four of these products were one-step processes using stannous chloride as the reducing agent; the fifth was based on an electrolytic reduction. In addition, we also assessed our own modification of the electrolytic method. We measured the radiochemical purity by precipitation with trichloroacetic acid and by gel filtration on a Biogel P4 column. In addition, we measured the clearance of radioactivity from the blood at frequent time intervals after intravenous injection. Each product was assessed in separate groups of six patients. The labelling efficiency of the one-step kits varied between 73 and 93% compared with 94 and 98% for the electrolytically labelled albumin. The blood clearance for all one-step kits was significantly faster than that obtained for the radiopharmaceuticals prepared by the electrolytic method. We conclude that HSA labelled with 99mTc by the electrolytic method is to be preferred.     

Technetium-99m labelled human serum albumin for ventriculography: a comparative evaluation of six labelling kits

In this study we have compared the characteristics of six labelling kits for the preparation of technetium-99m labelled human serum albumin (^99mTc-HSA) and evaluated the usefulness of the various ^99mTc-HSA preparations as blood pool tracer agents. The amount of the principal ingredients, i.e. HSA and stannous ions, varies largely between the studied kits and this is probably a reason for the observed differences in the labelling rate. Analysis of the reaction mixtures after labelling of the respective kits with ^99mTc showed in each preparation the presence of four to five radioactive components in variable relative amounts. The retention time of the main component on size-exclusion high-performance liquid chromatography (SEC-HPLC) was identical for all preparations. Biodistribution of the HPLC-isolated fractions was studied in mice. The components with the shortest and longest retention times on HPLC show poor retention in the plasma. The three intermediate fractions, including the principal peak, are initially retained relatively well in the blood (60%–70% of the injected dose after 10 min), but clearly to a lower degree than iodine-125 labelled HSA. Moreover, they diffuse out of the vascular compartment at a much higher rate than ¹²⁵I-HSA. The biological behaviour of the main component of the various preparations was clearly different, despite the identical retention time on SEC-HPLC. Study of the total preparations in mice and a rabbit showed that two of them are cleared rapidly from the blood and cannot be considered valuable blood pool tracers. Diffusion of the other preparations out of the blood is slower but also considerable and compromises their use for ventriculography.K.A. Verbeke is a Research Assistant for the Belgian National Fund for Scientific Research     

First evaluation of technetium-99m dimercaptopropionyl albumin as a possible tracer agent for ventriculography in a volunteer

Technetium-99m labelled red blood cells (^99mTc-RBCs) are the standard radiopharmaceutical for radionuclide ventriculography but suffer from some practical disadvantages such as risk of viral contamination and lengthy preparation (in vitro labelling) or poor labelling efficiency due to patient medication interactions (in vivo labelling). ^99mTc-labelled human serum albumin (HSA) is not really a valuable alternative as the activity diffuses too rapidly out of the vascular space due to the weak binding of the radionuclide. We have modified HSA by reaction with N-hydroxysuccinimidyl 2,3di(S-acetylmercapto)propionate (SAMP) to introduce a varying number of 2,3-dimercaptopropionyl (DMP) side chains. The resulting DMP-HSA can be rapidly labelled with ^99mTc at room temperature by simple addition of stannous ions and eluate of a ^99mTc generator. After evaluation in mice and rabbits, two different ^99mTc-DMP-HSA preparations — obtained after reaction of SAMP with albumin in a molar ratio of, respectively, 8:1 and 16:1 — were tested in a volunteer and compared to ^99mTcRBCs. The blood time-activity curves of the three preparations were quite similar but both ^99mTc-DMP-HSA preparations were excreted much less into the urine than ^99mTc-RBCs. Ventriculography was performed with the three tracer agents, each time with a 1-week interval. In the three studies, the heart was clearly visualized and the left and right ventricle could be easily delineated. The ejection fractions obtained after administration of the three preparations were similar. With both ^99mTc-DMP-HSA preparations the low activity in the spleen was a distinct advantage and facilitated delineation of the left ventricle. However, a slightly higher liver uptake was seen with ^99mTc-DMP-HSA 16:1, whereas the liver uptake of ^99mTc-DMP-HSA 8:1 and ^99mTc-RBCs was the same. These first results suggest that ^99mTc-DMP-HSA, and in particular ^99mTc-DMP-HSA 8:1, could be used for ventriculography as a practical and reliable alternative to ^99mTc-RBCs.     

The use of technetium-99m labelled heat-damaged red cells for the quantitative measurement of splenic function

A comparison of the rates of clearance of ⁵¹Cr labelled and ^99mTc labelled heat-damaged red cells in 25 patients and 4 control subjects is reported. Very little correlation was found between the clearance half-times of the two types of labelled cells when the cells were labelled with ^99mTc prior to heat damaging. The correlation was improved when the labelling step occurred after the cells had been damaged. Urinary excretion measurements revealed that the rate of excretion of ^99mTc could be as much as nine times that of the ⁵¹Cr label. ^99mTc labelled heat-damaged red cells were found not to be sufficiently stable a preparation for use in quantitative clearance studies.     

Can technetium-labelled millimicrospheres be used to measure Kupffer-cell function? An experimental study

It has been suggested that sodium pertechnetate ^99mTc millimicrospheres can be used to measure Kupffercell function. We studied animals and humans to show whether the clearance and catabolism of ^99mTc-labelled millimicrospheres can be used as a measure of Kupffer-cell function. Comparison with albumin ¹²⁵I-microaggregates clearance of human serum albumin failed to demonstrate that they can be used for this purpose. We suggest that their blood clearance is mainly an expression of liver blood flow.     

Pharmacokinetic Alteration of 99mTc-MAG3 using Serum Protein Binding Displacement Method

IntroductionWhen a radiopharmaceutical is simultaneously administered with a medicine that has high affinity for the same plasma protein, the radiopharmaceutical is released at higher concentrations in blood, leading to enhanced transfer into target tissues. This is known as the serum protein binding displacement method. In this study, we investigated the pharmacokinetic alteration of technetium-99m-labeled mercaptoacetylglycylglycylglycine (^99mTc-MAG3) using the serum protein binding displacement method.MethodsRat and human serum protein binding rates of ^99mTc-MAG3 were measured by ultrafiltration with or without displacers of human serum albumin (HSA) binding sites I and II (200 μM and 400 μM loading). Male Wistar rats were injected with ^99mTc-MAG3 (740 kBq/0.3 mL saline) via the tail vein, and biodistribution was assessed at 2, 5, 10 and 15 min. Dynamic whole-body images were obtained for ^99mTc-MAG3 (11.1 MBq/0.3 mL saline)-injected rats, with or without HSA displacers.Results^99mTc-MAG3 strongly bound to HSA (87.37%±2.13%). Using HSA site I displacers, the free fraction of ^99mTc-MAG3 increased significantly (1.20 to 1.47 times) when compared with controls. For biodistribution and imaging, rapid blood clearance was observed with bucolome (BCL) loading, which is an HSA site I displacer. With BCL loading, peak times for rat renograms were respectively shifted from 240 s to 110 s, and from 170 s to 120 s.ConclusionsWe found that ^99mTc-MAG3 bound to the HSA binding site I. It was confirmed that pharmacokinetic distribution of ^99mTc-MAG3 is altered by presence of BCL, which leads to increases in the free fraction of ^99mTc-MAG3, and BCL produced rapid blood clearance and fast peak times on rat renograms. The serum protein binding displacement method using ^99mTc-MAG3 and BCL, a safe displacer for humans, may be applicable to clinical study and lead to better diagnostic images with shorter waiting times and lower radiation doses for patients.     

Electrolytic 99mTcO4− reduction: A different pathway to obtain 99mTc-labelled compounds

Electrolytic reduction of ^99mTcO₄⁻ at inert electrodes to obtain ^99mTc cationic complexes and in vitro stability of labelled compounds were studied. Amines were used as neutral N-donor ligands and a systematic analysis of various parameters involved in the reduction process was performed.Usefulness of electrolytic reduction was proved as an alternative ^99mTc-labelling method. Its most important advantages are: production of complexes with a high radiochemical purity, negligible presence of red-hyd-^99mTc, lack of foreign materials, simplicity of development and possibility of further applications.     

Technetium-99m mercaptoalbumin as a potential substitute for technetium-99m labelled red blood cells

Technetium-99m labelled red blood cells (^99mTc-RBCs) are far superior to ^99mTc-labelled human serum albumin (^99mTc-HSA) for radionuclide ventriculography, but their labelling is more complex, time consuming and risk bearing (in vitro labelling) or suffers from interference by some medications (in vivo labelling). We have now modified HSA by the introduction of mercapto groups with the purpose of preparing stable and practical ^99mTc-mercaptoalbumin with long retention in the vascular system, that could replace ^99mTc-RBCs. HSA was incubated with N-succinimidyl S-acetylthioacetate (SATA) or N-succinimidyl 2,3-di(S-acetylthio) propionate (SATP) to introduce a chain containing one or two protected sulfhydryl groups on some of the lysine amino groups. After purification by size-exclusion chromatography (SEC), the mercapto groups were deprotected by incubation at alkaline pH or by treatment with hydroxylamine. The reaction products were used with or without SEC purification for direct or exchange labelling experiments with ^99mTc at neutral pH. SEC-HPLC was used to determine labelling yields and to isolate pure ^99mTc-mercaptoalbumin. Stable ^99mTc-mercaptoalbumin complexes could be formed in 90%–95% yield after coupling albumin with SATA or SATP in all molar ratios used followed by deacetylation in one of the mentioned conditions. The most favourable results were obtained after reaction of SATA or SATP with HSA in a 25: 1 ratio and deprotection with NH₂OH. The stability of the resulting ^99mTc-mercaptoacetyl-albumin (^99mTc-MAHSA) and ^99mTc-dimercaptopropionyl-albumin (^99mTcDMP-HSA) and their retention in vivo in plasma of mice and rabbits are clearly higher than that of conventional ^99mTc-HSA preparations. ^99mTc-DMP-HSA approaches the behaviour of ¹²⁵I-HSA quite well in both animal species. A preliminary study with ^99mTc-DMP-HSA in a volunteer showed a retention in the vascular compartment almost identical to that of ^99mTc-RBCs and clearly higher than that of a common ^99mTc-HSA preparation. The results indicate that these ^99mTc-mercaptoalbumins and especially ^99mTc-DMP-HSA are very promising as a practical alternative to ^99mTc-RBCs.K.A. Verbeke is a Research Assistant for the Belgian National Fund for Scientific Research     

Evaluation of potential heart affine compounds by use of the isolated perfused rat heart

^99mTc-organocation complexes were screened for heart affinity using the isolated perfused rat heart. The ^99mTc-I compounds are better extracted by the heart and retained in it than the ^99mTc-III compounds. ^99mTcCl₂(DMPE) ₂ ⁺ shows low uptake (15%) and fast washout, but no impairment of uptake after incubation with human serum albumin(HSA), with a modified preparation, initial uptake is not much better (18%) but washout is clearly diminished. ^99mTc(DMPE) ₃ ⁺ is taken up to a greater extent (33%) if administered solely, but when given in conjunction with HSA, uptake decreases to about 4% because of strong affinity of the complex to the protein, but this compound persists well in the tissue. Detergents like tween 80 and dioctylsulphosuccinate are able to cause higher uptake of ^99mTc(DMPE) ₃ ⁺ even in the presence of HSA, thus indicating a diminished binding of the compound to the protein. ^99mTc(CN-t-butyl) ₆ ⁺ shows very high uptake (80%) by the heart, which is only moderately diminished by addition of HSA (t0 38%), and persists excellently in the myocardium.Basic features of the compounds such as accumulation in and elimination from the myocardium as well as plasma protein binding and their mutual relationships are well reflected by the isolated heart model.     

Imaging of intraperitoneal tumors with technetium-99m GSA

^99mTc labeled galactosyl serum albumin (GSA) has been used clinically as a receptor-binding agent for the assessment of liver function. The aim of this study was to investigate the usefulness of^99mTc-GS A in intraperitoneal (i.p.) tumor imaging. A tumor model was established by i.p. inoculating nude mice with human ovarian cancer cell SHIN-3, or colon cancer cell LS180. Radiolabels were i.p. injected into the tumor-bearing mice and the biodistribution of radioactivity was examined. After administration,^99mTc-GSA rapidly accumulated in the tumor. The tumor uptake was 5.82-8.46 %ID/g from 30 min to 6 h after the injection. Radioactivity in the blood was very low, less than 0.3 %ID/g, resulting in high tumor-to-blood ratio. Tumors could be clearly seen by scintigraphic imaging. Accumulation of i.p.-injected^99mTc labeled human serum albumin (HSA) in i.p. tumors was similar to that of^99mTc-GSA, but radioactivity of^99mTc-HSA in the circulation was high, resulting in a significantly lower tumor-to-blood ratio. In conclusion,^99mTc-GSA, when i.p. injected, accumulated in i.p. tumors and cleared from circulation rapidly, which would make it useful for the imaging of i.p. tumors.     

The biological fate of sulphur colloid

The in-vivo behaviour of sulphur colloid has been investigated using colloids labelled with ³⁵S as well as ^99mTc. The rates of clearance of ³⁵S and ^99mTc from the blood, the rates of accumulation in liver and bone and the distribution of the two radioisotopes in various organs are all markedly different. The results demonstrate that although technetium is rapidly removed from the blood stream and primarily accumulated in the liver the colloid particles themselves are broken down in vivo with the release of sulphur.     

The behavior of99mTc-hexamethylpropyleneamineoxime (99mTc-HMPAO) in blood and brain

^99mTc-hexamethylpropyleneamineoxime (^99mTc-HMPAO) is a reagent for scanning cerebral blood flow. We investigated how^99mTc-HMPAO changed in the blood and brain. The^99mTc-HMPAO, which was prepared by adding of^99mTcO _- ⁴ to HMPAO and Sn(II), consisted of primary and secondary complexes, reduced hydrolyzed^99mTc, and^99mTc0pertechnetate. The percentage of the primary complex in^99mTc-HMPAO decreased with time after preparation. The primary complex converted to the secondary one very rapidly in the presence of plasma. When^99mTc-HMPAO was injected into patients,^99mTc activity was immediately partitioned in the plasma fraction, with approximately 60% in whole blood. In plasma,^99mTc was found to be associated with proteins such as albumin and globulin.^99mTc trapped in red cells was not washed out with either plasma or saline. Biodistribution studies showed that the less lipophilic compounds of^99mTc-HMPAO could not pass through the blood brain barrier (BBB), and therefore did not accumulate in the brain. The results of gel chromatography and equilibrium dialysis indicated that no specific^99mTc binding protein was present in the brain. Considering the instability of^99mTc-HMPAO in vivo, we proposed that the speed at which the primary complex converted to the less lipophilic compounds was important in allowing^99mTc-HMPAO to pass through the BBB and to be fixed in the brain.     

Anti-MUC1 aptamers: radiolabelling with Tc and biodistribution in MCF-7 tumour-bearing mice

IntroductionAptamers previously selected against the protein core (AptA) or the tumour glycosylated (AptB) MUC1 glycoprotein have been conjugated to MAG2 and labelled with ^99mTc, for the potential use as radiopharmaceuticals for diagnostic imaging of breast cancer.MethodsThe conjugation was achieved in high yield using standard peptide coupling reactions between an amino modification on the aptamer and the activated carboxylic group on the ligands. The retention of the affinity of the MAG2 modified AptA for the MUC1 protein core was confirmed using a fluorescent intercalator displacement binding assay. The labelled aptamers were separated from free ^99mTc using ultrafiltration and monitored by high-performance liquid chromatography at all stages, to ensure that only radiolabelled aptamers were produced. The biodistribution properties of the two aptamer-radionuclide conjugates were analysed in MCF-7 tumour bearing mice and compared.ResultsEfficient and convenient labelling of the two aptamers with ^99mTc was achieved as the last step of the synthesis (post-conjugation labelling). Both the aptamer-chelator conjugates had strong ^99mTc binding properties and the resulting complexes were stable in vivo, both in terms of nuclease degradation and leaking of the metal. The radiolabelled aptamers showed a high renal clearance and a high uptake in the intestine.ConclusionsAptA and AptB have been successfully conjugated in high yield to the ligand MAG2 and labelled with ^99mTc. The radiolabelled aptamers showed different tumour uptake and clearance, but will require further development prior to diagnostic use.     

The use of human albumin millimicrospheres tagged with 99mTc in the evaluation of the removal capacity of the reticuloendothelial system

Liver uptake kinetics of ^99mTc labelled millimicrospheres of human serum albumin (MM) was studied in 16 subjects. Every subject received four doses of MM intravenously. The uptake constant decreased progressively with increasing dose. The maximum liver removal capacity, a parameter which is independent of liver blood flow, was calculated according to the method of Iio and Wagner (1963). From these data we conclude that MM are taken up by the reticuloendothelial system (RES) with saturable kinetics, and they are suitable for clinical use to evaluate RES function in man.     

Pre-targeted immunodetection in glioma patients: tumour localization and single-photon emission tomography imaging of [99mTc]PnAO-biotin

The imaging of cerebral gliomas with radiolabelled monoclonal antibodies (MoAbs) has been previously reported. However, previous studies have been hampered by the drawback of a low tumour to non-tumour ratio. In order to overcome this problem we have developed a three-step pre-targeting method using the avidin-biotin system. The rationale of this technique consists in vivo labelling of biotinylated MoAbs targeted onto tumour deposits, when most of the unbound antibodies have been cleared from the bloodstream as avidin-bound complexes. The anti-tenascin MoAb BC2, specific for the majority of gliomas, was biotinylated and 1 mg was administered i.v. in 20 patients with histologically documented cerebral lesions. After 24–36 h, 5 mg avidin was injected i.v. followed 24 h later by a third i.v. injection of 0.2 mg PnAO-biotin labelled with 15–20 tnCi technetium-99m. No evidence of toxicity was observed. Whole-body biodistribution was measured at 20 min, 3 h and 5 h post-injection. [^99mTc]PnAO-bio-tin had a fast blood clearance and was primarily excreted through the biliary system. A dedicated single-photon emission tomography system was used to acquire brain tomographic images 1–2 h after the administration of [^99mTc]PnAO-biotin. Tumours were detected in 15/18 glioma patients with a tumour to non-tumour ratio of up 14:1. This three-step method, based on the sequential administration of anti-tenascin MoAb BC2, avidin and [^99mTc]PnAO-biotin, can support computed tomography or magnetic resonance imaging for the diagnosis and follow-up of patients with glioma. Further studies are required to evaluate the potential of this technique for therapeutic application.     

Isolation and labelling of human leukocytes with 99mTc

The role of the leukocyte isolation procedure on cell labelling with ^99mTc has been evaluated. Separation of leukocytes was performed by two procedures: (1) sedimentation on methyl cellulose, followed by discontinuous gradient centrifugation; (2) methyl cellulose sedimentation and hypotonic haemolysis of residual red blood cells. After washing the cells in saline and incubation with a stannous pyrophosphate agent, the leukocytes were labelled with 5–10 mCi ^99mTc. Procedure 1 gave a higher purity but lower recovery of polymorphonuclear leukocytes, and a minor contamination of red blood cells. ^99mTc labelling of cells was slightly more efficient with this method, probably due to the presence of red blood cells. Procedure 1 is recommended for in vitro studies on cell kinetics and procedure 2 is recommended for clinical use.     

Technetium-99m labelled hydrazinonicotinamido human non-specific polyclonal immunoglobulin G for detection of infectious foci: a comparison with two other technetium-labelled immunoglobulin preparations

Recently a new linker — hydrazinonicotinate (HYNIC) — was introduced for labelling of proteins and peptides with technetium-99m. HYNIC and other linkers have been used for labelling of human non-specific polyclonal immunoglobulin G (hIgG) with^99mTc for the detection of infections. In this study we compared the tissue distribution of three different^99mTc-hIgG preparations in groups of five Wistar rats with a focal intramuscular infection withStaphylococcus aureus. We compared^99mTc-HYNIC-hIgG with^99mTc-hIgG labelled via the so-called Schwarz method (reduction of disulphide bonds) and with the^99mTc-labelled commercially available Technescan-HIG. Unlike the HYNIC linker, in the two other labelling methods free sulph-hydryl groups are involved in the binding of^99mTc. High-performance liquid chromatography analysis of the labelled preparations and of plasma samples revealed aggregate or polymer formation in all three agents; this was least pronounced in the product labelled by means of the Schwarz method. The tested preparations did not show signs of degradation in vitro. The difference in linker chemistry was reflected in the tissue distribution. Thus the biodistribution of^99mTc-HYNIC-hIgG was significantly different from the distribution of the two other preparations: abscess (1.4%±0.2%ID/g), muscle, liver, spleen, plasma, lung, bone marrow, and small intestine concentrations were higher at 24 h p.i.; kidney uptake (1.19%±0.08%ID/g) was significantly lower. The abscess-to-plasma and the abscess-to-muscle ratios (0.5 and 11, respectively), however, were in the same range for the three preparations tested. Quantitative analysis of the scintigraphs revealed that the total body clearance of^99mTc-HYNIC-hIgG was significantly slower than for the other agents. The abscess uptake of^99mTc-HYNIC-hIgG as a percentage of the remaining body activity was significantly higher. Based on its high abscess uptake, its low uptake in the kidneys and the high percentage of its abscess uptake in relation to the remaining body activity, we conclude that^99mTc-HYNIC-hIgG seems superior to the two other preparations tested for the detection of infections.     

Rapid measurement of blood leakage during regional chemotherapy

In order to avoid complications after regional chemotherapy (isolated hyperthermic perfusion) of the extremities, rapid measurement of blood leakage from the extracorporeal to the systemic circulation is important. A method using technetium-99m in vivo red blood cell (RBC) labelling is reported that provides results within 3 min. Blood samples drawn from the systemic and the extracorporeal circulation were measured for ^99mTc activity using a mobile well counter, and the leakage values calculated. The mean result was 7.6%±6.5%/15 min (n=209). The corresponding flow rate was 100.2±85.7 ml/15 min (mean ± SD). The values for isolation perfusion of the upper and the lower extremities are compared. The leakage results using ^99mTc RBC labelling were correlated with other blood pool markers. Iodine-125 human serum albumin and indium-113 m transferrin were administered in subgroups of 4 and 19 patients simultaneously. Using linear regression, the coefficient of correlation was 0.72 for ^99mTc/^113mIn and 0.58 for ^99mTc/¹²⁵I. Comparison with the alternatives suggests that the rapid method of leakage measurement after ^99mTc RBC labelling can be considered one of the most practicable and reliable methods available.This paper is dedicated to Prof. E. Oberhausen, Homburg/Saar, on the occasion of his 65th birthday Correspondence to: C. Alexander     

A comparison of radiopharmaceutical labelling efficiency of chromatographic generator vs MEK extraction [99mTc]pertechnetate

Technetium-99m radiopharmaceuticals prepared for routine clinical use, were labelled with [^99mTc]pertechnetate obtained from either a commercial chromatographic generator or from a Winnipeg Health Sciences Centre semi-automated self-shielded methyl ethyl ketone extraction system. The [^99mTc]pertechnetate source for each ^99mTc radiopharmaceutical was selected at random over a 16 month period of time. The routine quality control data (silica-gel thin layer chromatography) was reviewed retrospectively, as an in vitro assessment of the quality of the [^99mTc]radiopharmaceutical prepared from each [^99mTc]pertechnetate source. Bone scans ([^99mTc]pyrophosphate) and wall motion studies ([^99mTc]red blood cells) were evaluated as an in vivo assessment of the [^99mTc]pertechnetate used to label the pyrophosphate or red blood cells. The in vitro studies indicated no difference in the labelling efficiency and radiochemical purity of the ^99mTc radiopharmaceuticals prepared from either source of [^99mTc]pertechnetate and there was also no difference observed in the image quality of either bone scans or wall motion studies obtained with either source of [^99mTc]pertechnetate.     

Failure in labelling of red blood cells with99mTc: Interaction between intravenous cannulae and stannous pyrophosphate

The effect of different methods of stannous pyrophosphate (SnPYP) administration on a modified in vivo technique for labelling red blood cells (RBCs) with^99mTc has been studied. Retention of^99mTc in the bloodstream was measured and parallel in vitro RBC labelling experiments were performed. After administration of SnPYP or a mixture of SnPYP and heparin via the metal needles of intravenous cannulae, the mean 30-min retentions of^99mTc were 96.8% (SD 8.6) and 95.3% (SD 12.4) respectively and the mean in vitro labelling efficiencies were 86.9% (SD 5.0) and 81.9% (SD 18.2) respectively. When the SnPYP was administered via teflon cannulae, the mean 30-min retention of^99mTc was 40.0% (SD 16.7) and the in vitro labelling efficiency was 18.7% (SD 27.0). Reduced RBC labelling and rapid clearance of^99mTc from the bloodstream were not explained by adsorption of stannous ion or pyrophosphate onto the teflon cannulae or retention of SnPYP in the cannulae. When labelling RBCs with^99mTc it is important that SnPYP is not injected via a teflon cannula.