In Vitro Evaluation of No-Carrier-Added Radiolabeled Cisplatin ([189, 191Pt]cisplatin) Emitting Auger Electrons

Precise quantitative evaluation of pharmacokinetics of cisplatin using a radio-platinum tracer in tumor-bearing mice

OBJECTIVE

The platinum-based antineoplastic drug cisplatin is commonly used for chemotherapy in clinics. This work aims to demonstrate a radio-platinum tracer is useful for precisely quantifying small amounts of platinum in pharmacokinetics studies.

METHODS

A cisplatin radiotracer (radio-cisplatin) was synthesized, and a comprehensive evaluation of cisplatin over 7 days after its intravenous injection into nude mice bearing a subcutaneous lung tumor (H460) was conducted.

RESULTS

A biphasic retention curve in the whole body and blood was observed [ T1/2 (α) = 1.14 h, T1/2 (β) = 5.33 days for the whole body, and T1/2 (α) = 23.9 min, T1/2 (β) = 4.72 days for blood]. The blood concentration decreased within 1 day after injection. Most of the intact cisplatin was excreted via the kidneys in the early time points, and a small part was distributed in tissues including tumors. The plasma protein binding rate of cisplatin increased rapidly after injection, and the protein-bound cisplatin remained in the blood longer than intact cisplatin. The peak uptake in H460 tumors was 4.7% injected dose per gram at 15 min after injection, and the area under the curve (AUC 0-7 days ) was approximately one-half to one-third of the AUC 0-7 days in the kidneys, liver, and bone, where some toxicity is observed in humans.

CONCLUSION

The radio-platinum tracer revealed the highly quantitative biodistribution of cisplatin, providing insights into the properties of cisplatin, including its adverse effects. The tracer enables a precise evaluation of pharmacokinetics for platinum-based drugs with high sensitivity.

Special radionuclide production activities – recent developments at QST and throughout Japan

National Institutes for Quantum Science and Technology (QST), formerly known as the National Institute of Radiological Sciences (NIRS), has been engaged in work on radiopharmaceutical science using cyclotrons since 1974. Eight pioneering researchers founded the basis of this field of research at NIRS, and to the present, many researchers and technicians have accumulated both scientific and technical achievements, as well as inherited the spirit of research. Besides, in recent years, we have developed production systems with AVF-930 cyclotron for various ‘non-standard’ radioisotopes applied in both diagnosis and therapy. Here, we review the past 50 years of our activities on radioisotope and radiopharmaceutical development, as well as more recent activities.

Cisplatin - A more Efficient Drug in Combination with Radionuclides?

AIM

The combination of conventional chemotherapeutic drugs with radionuclides or external radiation is discussed for a long period of time. The major advantage of a successful combination therapy is the reduction of severe side effects by decreasing the needed dose and simultaneously increasing therapeutic efficiency.

METHODS

In this study, pUC19 plasmid DNA was incubated with the cytostatic drug cisplatin and additionally irradiated with ^99mTc, ¹⁸⁸Re and ²²³Ra. To verify the contribution of possibly excited platinum atoms to the emission of Auger electrons we determined DNA damages, such as single- and double strand breaks.

RESULTS

The threshold concentration value of cisplatin, which was tolerated by pUC19 plasmid DNA was determined to be 18-24 nM. Nevertheless, even at higher dose values (>100 Gy) and simultaneous incubation of cisplatin to 200 ng plasmid DNA, no significant increase in the number of induced single- and double-strand breaks was obtained, compared to the damage solely caused by the radionuclides.

CONCLUSION

We thereby conclude that there is no direct dependence of the mechanism of strand break induction to the absence or presence of platinum atoms attached to the DNA. Reported increasing DNA damages in therapy approaches on a cellular level strongly depend on the study design and are mainly influenced by repair mechanisms in living cells. Nevertheless, the use of radioactive cisplatin, containing the Auger electron emitter ¹⁹¹Pt, ^193mPt or ^195mPt, is a bright prospect for future therapy by killing tumor cells combining two operating principles: a cytostatic drug and a radiopharmaceutical at the same time.

Development of Novel 191Pt-Labeled Hoechst33258: 191Pt Is More Suitable than 111In for Targeting DNA

This study aims to establish new labeling methods for no-carrier-added radio-Pt (¹⁹¹Pt) and to evaluate the in vitro properties of ¹⁹¹Pt-labeled agents compared with those of agents labeled with the common emitter ¹¹¹In. ¹⁹¹Pt was complexed with the DNA-targeting dye Hoechst33258 via diethylenetriaminepentaacetic acid (DTPA) or the sulfur-containing amino acid cysteine (Cys). The intranuclear fractions of ¹⁹¹Pt- and ¹¹¹In-labeled Hoechst33258 were comparable, indicating that the labeling for ¹⁹¹Pt via DTPA or Cys and the labeling for ¹¹¹In via DTPA worked equally well. ¹⁹¹Pt showed a DNA-binding/cellular uptake ratio of more than 1 order of magnitude greater than that of ¹¹¹In. [¹⁹¹Pt]Pt-Hoechst33258 labeled via Cys showed a higher cellular uptake than that labeled via DTPA, resulting in a very high DNA-binding fraction of [¹⁹¹Pt]Pt-Cys-Hoechst33258 and extensive DNA damage. Our labeling methods of radio-Pt, especially via Cys, promote the development of radio-Pt-based agents for use in Auger electron therapy targeting DNA.