CYTOTOXIC THERAPY REVISED: EXPLORATION OF INNOVATIVE (PRO)DRUGS IN PATIENT-DERIVED XENOGRAFTS OF SOFT TISSUE SARCOMA

Soft tissue sarcomas (STS) represent a heterogeneous group of rare, malignant tumors of mesenchymal origin, which comprises more than 80 histological subtypes. Despite their extensive histological and molecular variety, the majority of STS are treated in a rather uniform way, especially in advanced setting. For these patients, doxorubicin has remained the first-line standard-of-care ever since its first introduction in the 1970s. However, response rates to doxorubicin are only around 14% to 18% in unselected STS populations. Meanwhile, patients are put at risk of developing severe and irreversible anthracycline-related cardiotoxicity, not allowing to exceed a cumulative dose 550 mg/m2 over the lifetime of a patient. Consequently, first-line treatment is exhausted after administration of only 6-8 cycles and cannot be resumed in case of relapse, even in the few responding patients.

Although over the past few decades multiple clinical trials have been performed to test alternative systemic therapies for these patients, none succeeded to surpass the limited activity of single-agent doxorubicin, nor replace it as a first-line standard-of-care. An explanation can be found in the way these clinical trials were designed, including relatively small numbers of patients with a variety of STS subtypes, instead of a more rational biomarker-driven patient selection. Consequently, proper preclinical testing of new potentially more active and less toxic therapies for these patients remains an important priority.

In the course of this 4-year doctoral project, we established an additional 18 STS patient-derived xenograft (PDX) models by implantation of donor tissue from patients with STS. This brings our platform of STS PDX to 63 models of 16 different subtypes. All established models were thoroughly characterized by histological and/or molecular means and demonstrated the characteristics of the original donor tumor. To our knowledge, this platform is currently the most extensive academic platform of STS PDX available, and provides valuable preclinical research models to the sarcoma community.

Using the established STS PDX models, we performed in vivo experiments with two tetrapeptidic prodrugs of doxorubicin that are activated by enzymes expressed in the tumor tissue and/or microenvironment. In this way, doxorubicin is released more selectively at the tumor site, which reduces doxorubicin-related side effects and allows to achieve higher local doses of active drug. In all STS PDX models investigated, PhAc-ALGP-Dox or CBR-049 demonstrated at least equal or superior antitumor activity than compared to standard-of-care doxorubicin. CBR-050 demonstrated antitumor efficacy only in one STS PDX and was not superior to doxorubicin. For both experimental prodrugs, strong immunostaining for THOP1, the main activating enzyme of both compounds, was found to predict better antitumor efficacy. These results warrant further clinical evaluation of PhAc-ALGP-Dox in patients with advanced STS, ideally selected for high THOP1-expressing tumors.

Additionally, we explored the in vivo efficacy of enapotamab vedotin, an AXL-specific antibody drug conjugate (ADC). Despite the fact that two models responded with clinically relevant and complete tumor regressions, majority of models demonstrated only limited antitumor activity. These results are in line with the limited activity observed with enapotamab vedotin in the clinical trial performed in patients with solid tumors, which led to the discontinuation of the development of this compound. However, this trial did not select for patients with target expression, which raises concern about the interpretation of the efficacy signals.

Lastly, we explored pixantrone, a non-cardiotoxic derivative of doxorubicin already approved for advanced non-Hodgkin lymphoma, in malignant peripheral nerve sheath tumor (MPNST) PDX models. We observed at least equal or superior responses as compared with standard-of-care doxorubicin in all four MPNST PDX investigated. These results warrant further evaluation of pixantrone as potentially better and safer alternative of doxorubicin in MPNST, a very aggressive STS subtype that is mostly occurring in young patients.

In conclusion, we expanded our STS PDX platform and performed a rational selection of models for the early preclinical exploration of some novel treatments. We demonstrated promising in vivo efficacy of PhAc-ALGP-Dox, a cytotoxic doxorubicin prodrug, and pixantrone, a non-cardiotoxic derivative of doxorubicin. Based on these results, both drugs should be prioritized for clinical evaluation in patients with advanced STS.