PhD Defense | Kaat Spoormans | Tumor Absorbed Dose-Response Relationships in Targeted Radionuclide Therapy

Tumor Absorbed Dose-Response Relationships in Targeted Radionuclide Therapy - A Preclinical Investigation of Lutetium-177 and Terbium-161 Radiolabeled Peptides

Targeted radionuclide therapy (TRT) employs radiopharmaceuticals that are designed to localize radioactive isotopes near cancerous cells. These radioisotopes undergo a nuclear decay and thereby emit radiation that interacts with the surrounding tissue and induces lethal damage. The most suitable radioisotopes are selected with ideally a short range radiation type for a more local irradiation, a half-life that matches biological half-life of the vector and a stable daughter radionuclide. The amount of energy the radiation deposits per mass of tissue is referred to as the absorbed dose in Gray (Gy), and is related to the amount of damage that is inflicted. However, absorbed dose calculations are rather complex in TRT since they require nuclear imaging at multiple time points, combined with physical dose deposition simulations, and are therefore often omitted. 

The goal in TRT is to deliver a lethal radiation dose to the cancerous cells, while keeping the absorbed dose to healthy tissues below toxicity levels. However, due to the lack of standard applied dosimetry and longitudinal studies and limited radiobiological data, the absorbed dose-response relationships for both the tumor and healthy tissues are unknown for TRT. Consequently, all patients receive a fixed amount of injected activity for a fixed number of treatment cycles, despite inter-patient variations in kidney functioning, receptor expressions, tumor perfusion, tumor heterogeneity, body weight etc. Patients could largely benefit from more personalized treatments, either by safely increasing the absorbed dose to the tumor, or by reducing the absorbed dose to healthy tissues to avoid unnecessary toxicities. However, this would require both standard applied dosimetry as well as well established absorbed dose-response relationships, preferably including also a TRT-specific absorbed dose-response model, similar to the tumor control probability (TCP) models that are utilized during treatment planning in conventional external beam radiation therapy (EBRT).

Also at preclinical level, dosimetry is no standard practice yet. Nonetheless, preclinical absorbed dose-response relationships could reveal underlying radiobiological mechanisms, which remain undiscovered or show ambiguous results when considering only the administered activity. Furthermore, absorbed dose calculations would allow a more rigorous evaluation of new radiopharmaceuticals during their development. This brings us to the following aims for this PhD thesis: 

• Investigating absorbed dose-response relationships for TRT, and thereby exploring the impact of TRT-specific irradiation characteristics, such as the low absorbed dose-rate, the heterogeneous absorbed dose distribution and the use of particle radiation instead of gamma radiation as used in conventional EBRT.

• Evaluating and comparing 161Tb as a novel radioisotope for peptide receptor radionuclide therapy (PRRT) to 177Lu, the standard radioisotope that is currently used. Both isotopes are similar regarding their β−spectrum, half-life and chemical properties, while 161Tb emits additional Auger and internal conversion (IC) electrons compared to 177Lu, which could potentially increase the treatment efficacy.  

In chapter 4, we established absorbed dose-response relationships in vitro for the clonogenic survival of CA20948 rat pancreatic cells after treatment with [177Lu]Lu-DOTATATE, [161Tb]Tb-DOTATATE, [177Lu]Lu-DOTA-LM3 and [161Tb]Tb-DOTA-LM3. These absorbed dose calculations were refined in chapter 5 by using more accurate S-values based on a more realistic, virtual CA20948 cell model instead of assuming a standard geometry of spherical cells. Complementary, in chapter 6, we investigated tumor absorbed dose-response relationships in vivo by performing a dose-escalating efficacy study using [177Lu]Lu-DOTATATE and [161Tb]Tb-DOTATATE. Finally, chapter 6 linked together both the in vitro and in vivo absorbed dose-response relationships with an analytical tumor-growth-model, based on heterogeneous tumor absorbed dose distributions, determined from both ex vivo autoradiography and mouse-specific SPECT images. 

In vitro absorbed dose-calculations have shown a 360 % higher absorbed dose to the cell nucleus for [161Tb]Tb-DOTATATE compared to [177Lu]Lu-DOTATATE, while this was only a 30 % increase in absorbed dose at tumor level. This large difference was devoted to varying contributions of the short-range IC and longe-range β− electrons between the cellular and tumor level, which should be considered carefully when evaluating the potentials of [161Tb]Tb-DOTATATE. Moreover, it suggested a suitable application for the treatment of small metastases, for which the cross dose from the β− electrons will be minimal, but the IC electrons from 161Tb can still deliver a substation absorbed dose. 

Both at in vitro and in vivo level, we did not observe any difference in absorbed dose-response for [161Tb]Tb-DOTATATE compared to [177Lu]Lu-DOTATATE. This could be explained by the range of the Auger electrons which was too short to reach the radiosensitive cell nucleus from within the cytoplasm, and thereby did not increase the biological effectiveness with their high linear energy transfer (LET). However, when the internalizing [161Tb]Tb-DOTATATE was compared to the membrane bound [161Tb]Tb-DOTA-LM3 in vitro, we did observe an additional quadratic component in absorbed dose-response curve for the latter one, potentially due to additional cell membrane damage induced by the Auger electrons. However, this hypothesis requires further research, but indicates the importance of subcellular targeting when employing short-range radiation types such as Auger electrons. 

Finally, we developed an analytical tumor-growth-model that was able to predict the tumor doubling time based on the cellular absorbed dose-response relationships. While the model largely overestimated the tumor doubling times when assuming a homogeneous absorbed dose distribution, the predictions improved substantially after a correction for the heterogeneous absorbed dose distributions, which illustrated the potential extensive impact of absorbed dose heterogeneity on TRT efficacy. By performing accurate dosimetry both at in vitro and in vivo level, we were able to thoroughly investigate absorbed dose-response relationships within TRT. Our findings emphasized that absorbed dose heterogeneity, both at subcellular and tissue level, can have a profound effect on TRT efficacy. Furthermore, it allowed an in-depth comparison of the currently used radioisotopes 161Tb and 177Lu for PRRT, and identified a larger potential of 161Tb to treat small metastases, in addition to the importance of considering subcellular targeting when utilizing 161Tb. Finally, we established and experimentally verified an analytical tumor-growth-model, which might bring us one step closer toward individualized treatments. 

Promoter:

• Michel Koole (KU LEUVEN)

SCK CEN mentors:

• Melissa Crabbé
• Koen Vermeulen