Abstract 3972: Activity of MM-398, nanoliposomal irinotecan (nal-IRI), in Ewings family tumor xenografts is associated with high exposure of tumor to drug and high SLFN11 expression
Authors: Kang Min H. , Song Michael M. , Makena Monish , Lee Joo-Sang , Hall Connor P. , Hindle Ashly , Ko Winford , Paz Nancy , Fitzgerald Jonathan , Drummond Daryl C. , Triche Timothy J. , Reynolds C. Patrick 
Published: May 2015

Introduction: Pharmacokinetic properties of irinotecan limit tumor exposure to the active metabolite SN-38. MM-398, a nanoliposomal irinotecan (nal-IRI) developed to attempt to enhance delivery of irinotecan and SN-38 to tumor cells is being tested in clinical trials. We assessed the pharmacokinetics and anti-tumor activity of nal-IRI in comparison to the current clinical formulation of irinotecan (free irinotecan) in preclinical models of pediatric cancer in relationship to expression of the putative DNA/RNA helicase SLFN11.

Experimental Design: Plasma and tissue samples were analyzed by liquid chromatography-tandem mass spectrometry for irinotecan and SN-38. In vivo activity of free irinotecan and nal-IRI was compared in subcutaneous xenograft models (3 each) of Ewing's sarcoma family of tumors (EFT), neuroblastoma (NB), and rhabdomyosarcoma (RMS). SLFN11 mRNA expression (real-time RT-PCR) was correlated to in vitro cytotoxicity of SN-38 (by DIMSCAN) in 20 EWS, 13 RMS, and 20 NB cell lines. SLFN11 mRNA expression in primary tumors (10 EFT, 10 NB, and 39 RMS) was assessed by Affymetrix HuEx microarrays.

Results: Plasma and tumor concentrations of irinotecan and SN-38 were ∼ 10-fold higher for nal-IRI than for free irinotecan at 4h, 6h, and 24h post-intravenous injection (p&lt;0.01). In 7 of 9 xenograft models, nal-IRI significantly extended event-free survival (EFS) compared to controls, and for 6 of 9 models compared to irinotecan. Combining the 3 EFT models tested, 23 of 25 nal-IRI treated mice achieved complete responses for &gt; 100 days (88% EFS on day 120). EFS for mice with RMS (median 39 days) and NB (median 81 days) was significantly shorter (P&lt;0.001) than for EFT. SN-38 exposure in tumors from nal-IRI may be due to tumor-associated macrophage accumulation, but neither tumor infiltration of macrophages nor tumor SN-38 levels showed significant differences between EFT and RMS or NB xenografts. SLFN11 expression has been reported to correlate with sensitivity to DNA damaging agents and the median SLFN11 mRNA expression was &gt; 100-fold higher in EFT cell lines and &gt; 10-fold in EFT primary tumors compared with NB or RMS cell lines or primary tumors (P&lt;0.0001). Relative cytotoxic/inhibitory concentrations (rIC50) of SN-38 showed a significant inverse correlation with SLFN11 expression in 20 EWS cell lines (r=-0.63, P=0.004).

Conclusions: Nal-IRI demonstrated higher anti-tumor activity than free irinotecan in pediatric solid tumor xenografts, presumably due to increased systemic and tumor exposures to the SN-38 active metabolite. The high SLFN11 expression reported in EFT cell lines and the strong nal-IRI activity we have observed in EFT xenografts warrants testing in correlative clinical studies to determine if SLFN11 can reliably be used as a biomarker to predict nal-IRI activity in patients.

Citation Format: Min H. Kang, Michael M. Song, Monish Makena, Joo-Sang Lee, Connor P. Hall, Ashly Hindle, Winford Ko, Nancy Paz, Jonathan Fitzgerald, Daryl C. Drummond, Timothy J. Triche, C. Patrick Reynolds. Activity of MM-398, nanoliposomal irinotecan (nal-IRI), in Ewings family tumor xenografts is associated with high exposure of tumor to drug and high SLFN11 expression. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3972. doi:10.1158/1538-7445.AM2014-3972