Background
The tumor microenvironment (TME) not only depends on tumor type but also on tumor location and tissue type. Therefore, it may interfere with drug accessibility and response to antibody-based therapies in an organ-specific manner. We have established a human CD22-transgenic, systemic B cell lymphoma mouse model representing local resistance to CD22-targeted, recombinant immunotoxins (rIT) in lymph nodes. Hence, the aim of this study was to analyze drug accessibility in different organs and screen for possible combination therapies to overcome organ-specific resistance.
Materials and Methods
Primary murine, MYC-driven B cell lymphoma positive for human CD22 (MyC22-1/-2/-3) were injected i.v. in syngeneic mice. Mice were treated with unlabeled or fluorochrome-labeled rIT. Organ-specific tumor infiltration, immune infiltration, and drug homing was determined ex vivo in bone marrow (BM), spleen (SPL), and lymph nodes (LN) by flow cytometry.
Results
I.v. injection of three distinct B cell lymphoma clones MyC22-1/-2/-3 resulted in diffuse organ infiltration of BM, SPL, and LN showing organ-specific immune infiltration and histology. Despite phenotypic similarity among the lymphoma clones, anti-CD22 immunotoxins induced tumor clone- and organ-specific treatment responses. In contrast to major tumor regression in BM and SPL, sensitivity to rIT differed between LNs of the three models. While LNs of MyC22-1 showed strong response to rIT, LNs of MyC22-2 and MyC22-3 were resistant. To measure organ-specific drug distribution, a fluorochrome-labeled rIT was injected i.v. and maximal fluorochrome intensity was reached 6 h after injection. The overall signal was lower in LN compared to SPL and BM of tumor-bearing and tumor-free mice indicating reduced drug accessibility in LN per se. Despite similarly high signal intensity in SPL among all models, signal in LN of rIT-resistant MyC22-2 and MyC22-3 was strongly reduced compared to rIT-sensitive MyC22-1 and tumor-free mice suggesting tumor response correlates with organ-specific drug accessibility. To counteract local resistance, we screened potential rIT enhancers and identified two drugs that achieved complete remissions in combination with rIT also in LNs of resistant mice. Although both drugs enhanced tumor response in LN, only one of them increased drug accessibility. Substituting the enhancer of rIT accessibility with a second drug of the same mechanistic family similarly reproduced the observed effects.
Conclusions
Tumor responses to anti-CD22 immunotoxins strongly varied in a tumor clone- and also in an organ-specific manner. Response correlated with reduced drug accessibility in LN emphasizing the importance to study drugs in systemic tumor models. Impaired accessibility was reversed in combination with rIT enhancers. Distinct accessibility of other antibody-based therapeutics is currently being evaluated.
F. Gsottberger: None. K. Wendland: None. S. Petkovic: None. L. Nitschke: None. A. Mackensen: None. F. Müller: C. Other Research Support (supplies, equipment, receipt of drugs or other in-kind support); Modest; MedImmune/AstraZeneca. D. Speakers Bureau/Honoraria (speakers bureau, symposia, and expert witness); Modest; MedImmune/AstraZeneca.