An engineered bacterial reporter gene fusion for radiotheranostics

Project Abstract
The most important unmet need in oncology is to overcome the lack of effective therapy for advanced solid
tumors in adults and children. Despite major advances in immunotherapy, radiation therapy, precision medicine,
and nuclear medicine, the vast majority of advanced solid tumors presenting clinically today are still incurable.
Radiotheranostic therapies offer the tremendous advantages of precision medicine and patient selection over
other cancer treatment modalities but lead to objective responses in only 30-60% of patients. Innovations in
radiopharmaceutical therapy (RPT) to address the major barriers to consistent tumor control are sorely needed:
suboptimal drug delivery and lack of retention of radionuclides at the target site. For cancer, RPT is administered
as an unconjugated or chelated radionuclide or in combination with a delivery vehicle, such as a peptide or
antibody (radioimmunotherapy). This project aims to develop a highly versatile RPT platform that harnesses
engineered bacteria to concentrate therapeutic radionuclides in tumors. We hypothesize that an engineered
bacterial fusion protein can serve as an in vivo artificial receptor for a small radionuclide carrier (as anti-2,2′,2”,2”'-
(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic
acid
(DOTA)-radiohapten). This construct, DOTA-
binding Salmonella, can specifically colonize tumors following intravenous administration and is cleared via the
liver and spleen. Notably, bacteria do not colonize in the radiosensitive red bone marrow or kidneys, which are
typically organs-at-risk during RPT. Based on the known pharmacokinetics, biodistributions, and clearance
properties of engineered bacteria, the DOTA-radiohapten can be injected precisely at the time of peak bacterial
tumor-to-normal tissue accumulation ratios (eg, after 48 hours, tumor-to-spleen ratios of 104 are typical). The
intratumoral bacteria will capture the DOTA-radiohapten and plasma DOTA-radiohapten will be rapidly and
efficiently excreted from the body via the renal route.
This project has two Aims. In Aim 1 we will genetically engineer Salmonella to express surface-anchored DPB
characterize its functionality in vitro. In Aim 2, we will demonstrate the efficacy of our proposed radio-theranostic
treatment paradigm based on Salmonella-DPB + 86/90Y-DOTA in mouse tumor models. This strategy has several
specific advantages over other radioimmune approaches. The number of radiohapten (DOTA) binding sites per
gram of tumor has the potential to be orders of magnitude greater than the number of the surface-marker sites
on cancer cells. The strategy will produce unprecedented therapeutic indices for critical organs (tumor vs.
kidneys and bone marrow) because Salmonella are cleared from the blood hours after injection, and
accumulation in the kidneys is minimal. Salmonella,
engineering
with its high tumor specificity, deep tissue penetration, and
plasticity, make it a highly promising for RPT.Engineered Salmonella, combined with in vivo capture
of safe, non-immunogenic, bioorthogonal DOTA-radiohaptens, offer a highly promising engineered bacteria
radiotheranostic platform for oncology.

Grant Number: 1R21CA287211-01A1
NIH Institute/Center: NIH
Principal Investigator: Sarah Cheal