Abstract P6-11-02: Spatial distribution of veliparib measured by matrix assisted laser desorption ionization mass spectrometry in triple negative breast cancer tumors
Authors: Bartelink Imke H , Prideaux Brendan , Hann Byron , Krings Gregor , Coppe Jean-Phillippe , Swigart-Brown Lamorna , Lee Pei Rong Evelyn , Esserman Laura , Yee Douglas , Wolf Denise , Savic Rada M , van t Veer Laura 
Published: May 2015

Background: Veliparib, an inhibitor of Poly(ADP-ribose) polymerase (PARPi), in combination with DNA-damaging agents showed significant efficacy, especially in triple negative breast cancer (TNBC) patients. However, in I-SPY 2, approximately 42% of TNBC did not optimally respond to the veliparib based treatment. To exert therapeutic effect, drugs such as veliparib or the DNA damaging agent carboplatin must reach cancer cells at an adequate concentration. Differences within the microenvironment of the tumor (e.g. reduced vascular density and leaky endothelium) are responsible for variability in the uptake and distribution of the drug in the tumor. We hypothesize that heterogeneous or insufficient drug concentrations in the solid tumor lead to inadequate response to PARPi in some TNBC patients. First we performed a mouse xenograft study to determine the penetration and distribution of veliparib and carboplatin in TNBC tumors in mice.

Methods: 1*106 MDA.MB.231, HCC70 or MDA.MB.436 cells were injected bilaterally into mamamary fat pad of a total of 36 Beige SCID mice. When the tumors exceeded 200 mm3, veliparib (20mg/kg or 60mg/kg) or placebo was orally administered 3 times daily for 3 days + Carboplatin 60mg/kg on day 1+2. These doses and dosing schedules were estimated to result equivalent plasma and tumor concentrations in mice compared to the doses currently used in clinical trials. Mice were euthanized 3 hours after the last dose of veliparib and tumors, and muscle tissues were obtained. In addition, control tissues were spiked with a broad range of concentrations of veliparib. The spatial distribution of the drugs in the tumor tissue was measured using Matrix-assisted laser desorption/ ionization mass spectrometric imaging (MALDI). The protonated molecular ion at m/z 245.1 was used to construct 2D ion maps of the tissue showing relative quantitation of drug levels. H&E staining was performed to characterize the tumors.

Results: Veliparib in control tissues was detected at very low concentrations with a range of detection between 100 fmol-1nmol. After dosing, veliparib penetrates into the tumors and was detectable at both dose levels. However, drug distribution within the tumors was observed to be inhomogeneous. In spots where the drug accumulated, necrosis was observed. Veliparib accumulated in spots in the tumor and near the rim of the tumor. Differences between cell lines were observed, with the largest accumulation at the rim in BRCA mutated cells.

Conclusions: Veliparib can be measured using MALDI with good specificity and sensitivity and using concentrations equivalent to patients. TNBC tumors show largely heterogeneous distribution of PARPi, with accumulation in the pushing margings of the tumor. Future analyses will determine whether this heterogeneity in drug distribution explains variability in response to PARPi therapies.

Citation Format: Imke H Bartelink, Brendan Prideaux, Byron Hann, Gregor Krings, Jean-Phillippe Coppe, Lamorna Swigart-Brown, Pei Rong Evelyn Lee, Laura Esserman, Douglas Yee, Denise Wolf, Rada M Savic, Laura van t Veer. Spatial distribution of veliparib measured by matrix assisted laser desorption ionization mass spectrometry in triple negative breast cancer tumors [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-11-02.