Standardization and quality control of an in-house formulation of 99mTc-(V)-DMSA for imaging and assessment of tumour biology: Work in progress

Full text: Early diagnosis and effective treatment is the key for proper cancer control and nuclear techniques play a key role in differential diagnosis and ongoing follow up in these patients. Tumour imaging properties of various common radiopharmaceuticals have been utilised clinically with varied success. It must be realised that the radiochemical purity and stability of a formulation should be standardised in order to achieve clinically reproducible results. The different success and faliure rates may be due to this reason where one radiopharmaceutical is being used by different groups with sometimes contradictory findings. The objective of the present study is to reproduce an optimum simple method of in house formulation of 99mTc- (V)-DMSA from the available DMSA (III) kits and use it as a tumour imaging agent in the assessment of tumour biology. Two commercially available kits of DMSA was used. One was from the Board of Radiation and Isotope Technology (BRIT) India and the other was from Amersham (UK). Formulation of the BRIT kit was done by adding 7 mg of NaHCO3 in 0.2 ml of water for injection and 99mTcO4 simultaneously to the reaction vial. The kit contains 1 mg of DMSA, 0.3 mg of SnCl2·2H2O. Equal volume of air is withdrawn from the reaction vial. Mixing and incubation was done for 15 min. Formulation of the Amersham kit (1 mg DMSA, 0.42 mg of SnCl2 · 2H2O, 0.7 mg ascorbic acid, 2.9 mg sodium chloride and 50 mg inositol) was done by adding 0.2 ml 7.5 % w/v NaHCO3 + 99mTcO4. Mixing followed by incubation was done for 15 min. Radiochemical purity was determined by the method of Westera et al using TLC (T-6145 TLC pre coated plated silica gel with 254 nm fluorescent indicator on polyester supplied by Sigma /silica gel 60F 254 TLC aluminium strip supplied by Merck) with a solvent containing n-butanol acetic acid-water (3:2:3 v/v). Free pertechnetate levels were measured using TLC - SG in saline. Percent labeling of DMSA and free pertechnetate was calculated and the ratio of DMSA (III) and DMSA (V) was evaluated. These were carried out for a period of over 3 hrs. Initial observations show that the radiochemical purity of DMSA (V) for the Amersham kit is optimum at about 1 hr post formulation whereas that of the kit supplied by BRIT the same radiochemical purity is achieved in 15 minutes ( > 90% ) The stability of both the kits were checked upto 3 hrs and found to be stable. Amersham kit contains ascorbic acid which acts as a mild antioxidant and is added to the DMSA (III) kit to get 100% pure DMSA (III). The ascorbic acid stops tin from being oxidised which is thought to be responsible for the conversion of DMSA (III) to DMSA (V). To enable the oxidation of tin we tried bubbling of pure oxygen for 20 min.A high radiochemical purity was achieved in 15 min which remained stable at 3 hours. Normal human biodistribution studies showed predominantly renal pelvicalyceal excretion in a radiochemically pure preparation with physiological concentration in bladder.The cardiac blood pool concentration is significant at 1 hour which gradually washes away by 3-4 hours time. Physiological tracer concentration was also seen in the liver and ocassionally in spleen. A pilot study has been carried out so far in 4 patients of histopathologically proved lung carcinoma with the BRIT kit. Increased tracer concentration was seen immediately post injection. The target to non target ratio was maximum in 3 h post injection. Concentration in bone metastasis was optimum at 1 hour post injection. Proportionate washout of tracer from the cardiac blood pool helped in visualising tumour overlying the heart. These results and the preliminary clinical evaluation show that the BRIT kit formulation achieves a faster conversion to DMSA (V) and achieves good tumour concentration in a radiochemically pure preparation. > 90 % purity is what is desirable for optimum tumour concentration. Amersham kit formulation without oxygen bubbling requires a longer time for conversion to DMSA (V). With the help of oxygen bub bling desired readiochemical purity is achieved in 15 mins. Thus we have been able to standardise and reproduce an optimum formulation of DMSA (V) which concentrates in the tumour in-vivo. Clinical evaluation will continue side by side with quality control studies and further comprehensive laboratory as well as clinical data will be presented in the meeting. (author)