Shubhangi Agarwal1, Troy Kozlowski1, Rohini Vidya Shankar1, Landon J. Inge2, and Vikram D. Kodibagkar1
1SBHSE, Arizona State University, Tempe, AZ, United States, 2St. Joseph's Hospital and Medical Center, Phoenix, AZ, United States
Synopsis
Solid tumors have hypoxic foci that be targeted using hypoxia
activated/targeting drugs. Utilizing quantitative MR oximetry techniques such
as the PISTOL (Proton Imaging of Siloxanes to map Tissue Oxygenation Levels)
technique could allow for patient stratification for personalized therapy. This
study uses the PISTOL technique to evaluate how the hypoxia activated drug TPZ
(Tirapazamine) depends on and affects the oxygenation and edema fraction of
epidermoid carcinoma (A431) and non-small cell lung cancer (H1975).
Surprisingly, TPZ was more effective on the H1975 tumors than A431, in
spite of higher pre-treatment pO2 levels, potentially due to
perfusion-related differences in tumor drug delivery.Purpose
Hypoxia promotes metastasis, proliferation,
resistance to chemo and radiotherapy and has been the focus for development of
novel hypoxia activating/targeting drugs [1-3]. Targeted, non-invasive in vivo
imaging of hypoxia has the potential assist in patient stratification for
incorporation of these drugs into a personalized therapeutic regimen. MRI
provides a powerful platform for assessing hypoxia [5-6] and recently a new pO2
measuring technique PISTOL (Proton Imaging of Siloxanes to map Tissue
Oxygenation Levels [7]) was developed for oximetry. In the present study,
PISTOL was used to determine the changes in oxygenation of tumor in
pre-clinical models of cancer in response to hypoxia activated therapeutic
intervention.
Methods
In-vivo
studies were carried out on cohorts of nu\nu mice (n=6) implanted
subcutaneously with 2x106 cells of NCI-H1975 (NSCLC) and A431
(epidermoid carcinoma) tumor cell line in the right thigh. Tirapazamine (TPZ,
hypoxia activated DNA damaging agent, 60 µmol/kg [8]) was administered in 5%
dextrose (control) solution intraperitoneally. The PISTOL sequence was used to
obtain pO2 maps every 5th day from day-0 (treatment date, ~0.3 cc
tumor volume) till day 15. The tumor was delineated on a T2 wt image and
segmented into periphery and central regions (outer and inner 50% voxels,
respectively) and pO2 maps for both the regions were also computed
and compared. The
edema fraction of the tumors was computed by analyzing the T2 weighted images
with a bi-exponential model. Standard t-tests were conducted for statistical
analysis and p<0.05 was considered significant.
Results
The
normalized mean tumor volume for TPZ treated H1975 tumors was significantly
lower than the control at day 15 while TPZ treated A431 tumors were comparable
to the control tumors (fig.1). Significant difference was also observed between
control A431 and H1975 tumor volumes. The mean volume doubling time for
untreated A431 tumors was 6.8 ± 0.5 days while for those treated with TPZ it
was about 7.1± 0.7 days. The corresponding doubling times
for H1975 tumors were 5.3 ± 0.4 days (control) and 8 ± 1 days (treated)
respectively. The mean baseline (day 0) pO2 for A431 cohort (24 ± 14 torr) was found to be significantly lower than the
H1975 cohort’s pO2 (42 ± 15 torr). The comparisons between pO2 of
tumor center showed a significantly lower pO2 in the A431 tumors (21
± 16 torr) as compared to H1975 (51 ± 30 torr). TPZ treated H1975 cohorts had a
significant decrease in the mean oxygenation at day 5, 10 and 15 from day 0,
while A431 cohort showed significant decrease in the peripheral region’s
oxygenation only at day 10. A431 cohort showed a significant increase in
central region’s oxygenation from day 0 to day 15 while the overall oxygenation
was significantly higher from day 5 (17 ± 6 torr) to day 15 (34 ± 22 torr).
H1975 cohort had a significant decrease in the overall oxygenation at day 5 and
was at 24 ± 13 torr (44 ± 13 torr (day 0). Upon analysis of the central and
peripheral region separately, it was observed that both the regions had a
significant decrease in oxygenation with peripheral region’s oxygenation
showing a stronger significance than central region. At day 5 the control H1975
tumors (0.15 ± 0.02) had significantly higher edema fraction than the TPZ
treated H1975 tumors (0.13 ± 0.01). Again at day 10 the control H1975 tumors (0.17
± 0.04) had significantly higher edema fraction than the TPZ treated H1975
tumors (0.12 ± 0.03).
Discussion
In the current study the results indicate that the relationship between
tumor pO2 and efficacy of hypoxia activated pro-drugs is complicated
and depends on several factors.
Statistical comparison between normalized mean volumes across the
cohorts demonstrates surprisingly that TPZ is more effective in H1975 as
compared to A431 even though the baseline oxygenation is lower for A431 tumors.
The successful treatment of the H1975 cell line resulted in a lower edema
fraction. TPZ prodrug is reduced to its toxic counterpart by intracellular
reductases in a two-step process. Analysis for the presence of these reductases
in A431 and H1975 cell lines would assist in analyzing the response and
correlate with the results obtained. Hypoxia activated pro-drugs need to be
able to diffuse into the regions with severe hypoxia for their metabolic
conversion to the toxic product. Overall higher levels of baseline hypoxia may
indicate poorer perfusion as well, leading to lower tumor transport of the
pro-drug and hence reduced efficacy. Future and ongoing studies are focused on
correlation with pimonidazole staining among the cohorts.
Acknowledgements
No acknowledgement found.References
[1] Wilson, W.R. and M.P. Hay, Nat Rev Cancer, 2011. 11(6): p. 393-410. [2] Shannon, A.M.,
et al. Cancer Treat Rev, 2003. 29(4): p. 297-307 [3] Stewart, G.D., et al. BJU
Int, 2010. 105(1): p. 8-13. [5] Zhang Z et
al Magn Reson Med 2014; 71: 561-9 [6] Mason R.P. et al Q J Nucl Med Mol Imaging. Jun 2010; 54(3): 259–280. [7]
Kodibagkar VD et al, NMR Biomed. 2008; 21: 899–907. [8] Reddy S.B .et
al, Expert Opin Investig Drugs. 2009.