Comparison of [18F]-Tracers in Various Experimental Tumor Models by PET Imaging and Identification of an Early Response Biomarker for the Novel Microtubule Stabilizer Patupilone

O-(2-[18F]-Fluoroethyl)-L-Tyrosine (FET) in Neurooncology: A Review of Experimental Results

In recent years, PET using radiolabelled amino acids has gained considerable interest as an additional tool besides MRI to improve the diagnosis of cerebral gliomas and brain metastases. A very successful tracer in this field is O-(2-[18F]fluoroethyl)-L-tyrosine (FET) which in recent years has replaced short-lived tracers such as [11C]-methyl-L-methionine in many neuro-oncological centers in Western Europe. FET can be produced with high efficiency and distributed in a satellite concept like 2- [18F]fluoro-2-deoxy-D-glucose. Many clinical studies have demonstrated that FET PET provides important diagnostic information regarding the delineation of cerebral gliomas for therapy planning, an improved differentiation of tumor recurrence from treatment-related changes and sensitive treatment monitoring. In parallel, a considerable number of experimental studies have investigated the uptake mechanisms of FET on the cellular level and the behavior of the tracer in various benign lesions in order to clarify the specificity of FET uptake for tumor tissue. Further studies have explored the effects of treatment related tissue alterations on tracer uptake such as surgery, radiation and drug therapy. Finally, the role of blood-brain barrier integrity for FET uptake which presents an important aspect for PET tracers targeting neoplastic lesions in the brain has been investigated in several studies. Based on a literature research regarding experimental FET studies and corresponding clinical applications this article summarizes the knowledge on the uptake behavior of FET, which has been collected in more than 30 experimental studies during the last two decades and discusses the role of these results in the clinical context.

Continuous low-dose infusion of patupilone increases the therapeutic index in mouse and rat tumour models

Patupilone is a microtubule-targeted cytotoxic agent with clinical efficacy, but causes diarrhoea in more than 80% of patients. The efficacy and tolerability of patupilone delivered continuously by subcutaneous (s.c.) mini-pumps [(mini-pump dose (MPD)] or by intravenous bolus administration [intravenous bolus dose (IVBD)] were compared preclinically to determine whether the therapeutic index could be improved. The antiproliferative potency in vitro of patupilone was determined by measuring total cell protein. Tumours were grown s.c. in rats (A15) or nude mice (KB31, KB8511) or intracranially in nude mice (NCI-H460-Luc). Efficacy was monitored by measuring tumour volumes, bioluminescence or survival. Toxicity was monitored by body weight and/or diarrhoea. Total drug levels in blood, plasma, tissues or dialysates were quantified ex-vivo by liquid chromatography-mass spectroscopy/mass spectroscopy. Patupilone was potent in vitro with GI50s of 0.24-0.28 nmol/l and GI90s of 0.46-1.64 nmol/l. In rats, a single IVBD of patupilone dose dependently inhibited the growth of A15 tumours, but also caused dose-dependent body weight loss and diarrhoea, whereas MPD achieved similar efficacy, but no toxicity. In mice, MPD showed efficacy similar to that of IVBD against KB31 and KB8511 tumours, but with reduced toxicity. In a mouse intracranial tumour model, IVBD was more efficacious than MPD, consistent with patupilone concentrations in the brain. MPD provided constant plasma levels, whereas IVBD had very high C0/Cmin ratios of 70-280 (rat) or 8000 (mouse) over the dosing cycle. Overall, the correlation of plasma and tumour levels with response indicated that a Cave of at least GI90 led to tumour stasis. Continuous low concentrations of patupilone by MPD increased the therapeutic index in s.c. rodent tumour models compared with IVBD by maintaining efficacy, but reducing toxicity.

18FDG, [18F]FLT, [18F]FAZA, and 11C-methionine are suitable tracers for the diagnosis and in vivo follow-up of the efficacy of chemotherapy by miniPET in both multidrug resistant and sensitive human gynecologic tumor xenografts

Expression of multidrug pumps including P-glycoprotein (MDR1, ABCB1) in the plasma membrane of tumor cells often results in decreased intracellular accumulation of anticancer drugs causing serious impediment to successful chemotherapy. It has been shown earlier that combined treatment with UIC2 anti-Pgp monoclonal antibody (mAb) and cyclosporine A (CSA) is an effective way of blocking Pgp function. In the present work we investigated the suitability of four PET tumor diagnostic radiotracers including 2-[(18)F]fluoro-2-deoxy-D-glucose ((18)FDG), (11)C-methionine, 3'-deoxy-3'-[(18)F]fluorothymidine ((18)F-FLT), and [(18)F]fluoroazomycin-arabinofuranoside ((18)FAZA) for in vivo follow-up of the efficacy of chemotherapy in both Pgp positive (Pgp(+)) and negative (Pgp(-)) human tumor xenograft pairs raised in CB-17 SCID mice. Pgp(+) and Pgp(-) A2780AD/A2780 human ovarian carcinoma and KB-V1/KB-3-1 human epidermoid adenocarcinoma tumor xenografts were used to study the effect of the treatment with an anticancer drug doxorubicin combined with UIC2 and CSA. The combined treatment resulted in a significant decrease of both the tumor size and the accumulation of the tumor diagnostic tracers in the Pgp(+) tumors. Our results demonstrate that (18)FDG, (18)F-FLT, (18)FAZA, and (11)C-methionine are suitable PET tracers for the diagnosis and in vivo follow-up of the efficacy of tumor chemotherapy in both Pgp(+) and Pgp(-) human tumor xenografts by miniPET.

Tumour T1 changes in vivo are highly predictive of response to chemotherapy and reflect the number of viable tumour cells--a preclinical MR study in mice

Background

Effective chemotherapy rapidly reduces the spin–lattice relaxation of water protons (T₁) in solid tumours and this change (ΔT₁) often precedes and strongly correlates with the eventual change in tumour volume (TVol). To understand the biological nature of ΔT₁, we have performed studies in vivo and ex vivo with the allosteric mTOR inhibitor, everolimus.

Methods

Mice bearing RIF-1 tumours were studied by magnetic resonance imaging (MRI) to determine TVol and T₁, and MR spectroscopy (MRS) to determine levels of the proliferation marker choline and levels of lipid apoptosis markers, prior to and 5 days (endpoint) after daily treatment with vehicle or everolimus (10 mg/kg). At the endpoint, tumours were ablated and an entire section analysed for cellular and necrotic quantification and staining for the proliferation antigen Ki67 and cleaved-caspase-3 as a measure of apoptosis. The number of blood-vessels (BV) was evaluated by CD31 staining. Mice bearing B16/BL6 melanoma tumours were studied by MRI to determine T₁ under similar everolimus treatment. At the endpoint, cell bioluminescence of the tumours was measured ex vivo.

Results

Everolimus blocked RIF-1 tumour growth and significantly reduced tumour T₁ and total choline (Cho) levels, and increased polyunsaturated fatty-acids which are markers of apoptosis. Immunohistochemistry showed that everolimus reduced the %Ki67⁺ cells but did not affect caspase-3 apoptosis, necrosis, BV-number or cell density. The change in T₁ (ΔT₁) correlated strongly with the changes in TVol and Cho and %Ki67⁺. In B16/BL6 tumours, everolimus also decreased T₁ and this correlated with cell bioluminescence; another marker of cell viability. Receiver-operating-characteristic curves (ROC) for everolimus on RIF-1 tumours showed that ΔT₁ had very high levels of sensitivity and specificity (ROC_AUC = 0.84) and this was confirmed for the cytotoxic patupilone in the same tumour model (ROC_AUC = 0.97).

Conclusion

These studies suggest that ΔT₁ is not a measure of cell density but reflects the decreased number of remaining viable and proliferating tumour cells due to perhaps cell and tissue destruction releasing proteins and/or metals that cause T₁ relaxation. ΔT₁ is a highly sensitive and specific predictor of response. This MRI method provides the opportunity to stratify a patient population during tumour therapy in the clinic.

Interplay of choline metabolites and genes in patient-derived breast cancer xenografts

Introduction

Dysregulated choline metabolism is a well-known feature of breast cancer, but the underlying mechanisms are not fully understood. In this study, the metabolomic and transcriptomic characteristics of a large panel of human breast cancer xenograft models were mapped, with focus on choline metabolism.

Methods

Tumor specimens from 34 patient-derived xenograft models were collected and divided in two. One part was examined using high-resolution magic angle spinning (HR-MAS) MR spectroscopy while another part was analyzed using gene expression microarrays. Expression data of genes encoding proteins in the choline metabolism pathway were analyzed and correlated to the levels of choline (Cho), phosphocholine (PCho) and glycerophosphocholine (GPC) using Pearson’s correlation analysis. For comparison purposes, metabolic and gene expression data were collected from human breast tumors belonging to corresponding molecular subgroups.

Results

Most of the xenograft models were classified as basal-like (N = 19) or luminal B (N = 7). These two subgroups showed significantly different choline metabolic and gene expression profiles. The luminal B xenografts were characterized by a high PCho/GPC ratio while the basal-like xenografts were characterized by highly variable PCho/GPC ratio. Also, Cho, PCho and GPC levels were correlated to expression of several genes encoding proteins in the choline metabolism pathway, including choline kinase alpha (CHKA) and glycerophosphodiester phosphodiesterase domain containing 5 (GDPD5). These characteristics were similar to those found in human tumor samples.

Conclusion

The higher PCho/GPC ratio found in luminal B compared with most basal-like breast cancer xenograft models and human tissue samples do not correspond to results observed from in vitro studies. It is likely that microenvironmental factors play a role in the in vivo regulation of choline metabolism. Cho, PCho and GPC were correlated to different choline pathway-encoding genes in luminal B compared with basal-like xenografts, suggesting that regulation of choline metabolism may vary between different breast cancer subgroups. The concordance between the metabolic and gene expression profiles from xenograft models with breast cancer tissue samples from patients indicates that these xenografts are representative models of human breast cancer and represent relevant models to study tumor metabolism in vivo.

Ga[NO2A-N-(α-amino)propionate] chelates: synthesis and evaluation as potential tracers for 68Ga PET

A reversible pH-trigged N₃O₃⇆N₄O₂ coordination isomerism was demonstrated for the Ga[NO2A-N-(α-amino)propionate] chelate in the pH range 4–6.

The enhanced in vivo activity of the combination of a MEK and a PI3K inhibitor correlates with [18F]-FLT PET in human colorectal cancer xenograft tumour-bearing mice

Combined targeting of the MAPK and PI3K signalling pathways in cancer may be necessary for optimal therapeutic activity. To support clinical studies of combination therapy, 3′-deoxy-3′-[¹⁸F]-fluorothymidine ([¹⁸F]-FLT) uptake measured by Positron Emission Tomography (PET) was evaluated as a non-invasive surrogate response biomarker in pre-clinical models. The in vivo anti-tumour efficacy and PK-PD properties of the MEK inhibitor PD 0325901 and the PI3K inhibitor GDC-0941, alone and in combination, were evaluated in HCT116 and HT29 human colorectal cancer xenograft tumour-bearing mice, and [¹⁸F]-FLT PET investigated in mice bearing HCT116 xenografts. Dual targeting of PI3K and MEK induced marked tumour growth inhibition in vivo, and enhanced anti-tumour activity was predicted by [¹⁸F]-FLT PET scanning after 2 days of treatment. Pharmacodynamic analyses using the combination of the PI3K inhibitor GDC-0941 and the MEK inhibitor PD 0325901 revealed that increased efficacy is associated with an enhanced inhibition of the phosphorylation of ERK1/2, S6 and 4EBP1, compared to that observed with either single agent, and maintained inhibition of AKT phosphorylation. Pharmacokinetic studies indicated that there was no marked PK interaction between the two drugs. Together these results indicate that the combination of PI3K and MEK inhibitors can result in significant efficacy, and demonstrate for the first time that [¹⁸F]-FLT PET can be correlated to the improved efficacy of combined PI3K and MEK inhibitor treatment.

PET imaging of proliferation with pyrimidines

Several new tracers are being developed for use with PET to assess pathways that are altered in cancers, including energy use, cellular signaling, transport, and proliferation. Because increased proliferation is a hallmark of many cancers, several tracers have been tested to track the DNA synthesis pathway. Thymidine, which is incorporated into DNA but not RNA, has been used in laboratory studies to measure tumor growth. Because thymidine labeled with (11)C undergoes rapid biologic degradation and has a short physical half-life, tracers labeled with (18)F have been preferred in PET imaging. One such tracer is (18)F-labeled 3'-deoxy-3'-fluorothymidine ((18)F-FLT). (18)F-FLT is trapped after phosphorylation by thymidine kinase 1, whose expression is increased in replicating cells. Several studies on breast, lung, and brain tumors have demonstrated that retention of (18)F-FLT correlated with tumor proliferation. Although (18)F-FLT has been used to image and stage several tumor types, the standardized uptake value is generally lower than that obtained with (18)F-FDG. (18)F-FLT can be used to image many areas of the body, but background uptake is high in the liver, marrow, and renal system, limiting use in these organs. (18)F-FLT PET imaging has primarily been studied in the assessment of treatment response. Rapid declines in (18)F-FLT retention within days to weeks have been demonstrated in several tumor types treated with cytotoxic drugs, targeted agents, and radiotherapy. Further work is ongoing to validate this approach and determine its utility in the development of new drugs and in the clinical evaluation of standard treatment approaches.

Preclinical molecular imaging using PET and MRI

Molecular imaging fundamentally changes the way we look at cancer. Imaging paradigms are now shifting away from classical morphological measures towards the assessment of functional, metabolic, cellular, and molecular information in vivo. Interdisciplinary driven developments of imaging methodology and probe molecules utilizing animal models of human cancers have enhanced our ability to non-invasively characterize neoplastic tissue and follow anti-cancer treatments. Preclinical molecular imaging offers a whole palette of excellent methodology to choose from. We will focus on positron emission tomography (PET) and magnetic resonance imaging (MRI) techniques, since they provide excellent and complementary molecular imaging capabilities and bear high potential for clinical translation. Prerequisites and consequences of using animal models as surrogates of human cancers in preclinical molecular imaging are outlined. We present physical principles, values and limitations of PET and MRI as molecular imaging modalities and comment on their high potential to non-invasively assess information on hypoxia, angiogenesis, apoptosis, gene expression, metabolism, and cell trafficking in preclinical cancer research.

An evaluation of 2-deoxy-2-[18F]fluoro-D-glucose and 3'-deoxy-3'-[18F]-fluorothymidine uptake in human tumor xenograft models

PURPOSE

The aim of this study is to assess the variability of 2-deoxy-2-[(18)F]fluoro-D: -glucose ([(18)F]-FDG) and 3'-deoxy-3'-[(18)F]-fluorothymidine ([(18)F]-FLT) uptake in pre-clinical tumor models and examine the relationship between imaging data and related histological biomarkers.

PROCEDURES

[(18)F]-FDG and [(18)F]-FLT studies were carried out in nine human tumor xenograft models in mice. A selection of the models underwent histological analysis for endpoints relevant to radiotracer uptake. Comparisons were made between in vitro uptake, in vivo imaging, and ex vivo histopathology data using quantitative and semi-quantitative analysis.

RESULTS

In vitro data revealed that [1-(14)C]-2-deoxy-D: -glucose ([(14)C]-2DG) uptake in the tumor cell lines was variable. In vivo, [(18)F]-FDG and [(18)F]-FLT uptake was highly variable across tumor types and uptake of one tracer was not predictive for the other. [(14)C]-2DG uptake in vitro did not predict for [(18)F]-FDG uptake in vivo. [(18)F]-FDG SUV was inversely proportional to Ki67 and necrosis levels and positively correlated with HKI. [(18)F]-FLT uptake positively correlated with Ki67 and TK1.

CONCLUSION

When evaluating imaging biomarkers in response to therapy, the choice of tumor model should take into account in vivo baseline radiotracer uptake, which can vary significantly between models.

Development of radiotracers for oncology--the interface with pharmacology

There is an increasing role for positron emission tomography (PET) in oncology, particularly as a component of early phase clinical trials. As a non-invasive functional imaging modality, PET can be used to assess both pharmacokinetics and pharmacodynamics of novel therapeutics by utilizing radiolabelled compounds. These studies can provide crucial information early in the drug development process that may influence the further development of novel therapeutics. PET imaging probes can also be used as early biomarkers of clinical response and to predict clinical outcome prior to the administration of therapeutic agents. We discuss the role of PET imaging particularly as applied to phase 0 studies and discuss the regulations involved in the development and synthesis of novel radioligands. The review also discusses currently available tracers and their role in the assessment of pharmacokinetics and pharmacodynamics as applied to oncology.

Positron Emission Tomography Imaging of Meningioma in Clinical Practice

Meningiomas represent about 20% of intracranial tumors and are the most frequent nonglial primary brain tumors. Diagnosis is based on computed tomography (CT) and magnetic resonance imaging (MRI). Mainstays of therapy are surgery and radiotherapy. Adjuvant chemotherapy is tested in clinical trials of phase II. Patients are followed clinically by imaging. However, classical imaging modalities such as CT and MRI have limitations. Hence, we need supplementary imaging tools. Molecular imaging modalities, especially positron emission tomography (PET), represent promising new instruments that are able to characterize specific metabolic features. So far, these modalities have only been part of limited study protocols, and their impact on clinical routine management is still under investigation. It may be expected that their extended use will provide new aspects about meningioma imaging and biology.

In the present article, we summarize PET imaging for meningiomas based on a thorough review of the literature. We discuss and illustrate the potential role of PET imaging in the clinical management of meningiomas. Finally, we indicate current limitations and outline directions for future research.

Gastrin-releasing peptide receptor-based targeting using bombesin analogues is superior to metabolism-based targeting using choline for in vivo imaging of human prostate cancer xenografts

Purpose

Prostate cancer (PC) is a major health problem. Overexpression of the gastrin-releasing peptide receptor (GRPR) in PC, but not in the hyperplastic prostate, provides a promising target for staging and monitoring of PC. Based on the assumption that cancer cells have increased metabolic activity, metabolism-based tracers are also being used for PC imaging. We compared GRPR-based targeting using the ⁶⁸Ga-labelled bombesin analogue AMBA with metabolism-based targeting using ¹⁸F-methylcholine (¹⁸F-FCH) in nude mice bearing human prostate VCaP xenografts.

Methods

PET and biodistribution studies were performed with both ⁶⁸Ga-AMBA and ¹⁸F-FCH in all VCaP tumour-bearing mice, with PC-3 tumour-bearing mice as reference. Scanning started immediately after injection. Dynamic PET scans were reconstructed and analysed quantitatively. Biodistribution of tracers and tissue uptake was expressed as percent of injected dose per gram tissue (%ID/g).

Results

All tumours were clearly visualized using ⁶⁸Ga-AMBA. ¹⁸F-FCH showed significantly less contrast due to poor tumour-to-background ratios. Quantitative PET analyses showed fast tumour uptake and high retention for both tracers. VCaP tumour uptake values determined from PET at steady-state were 6.7 ± 1.4%ID/g (20–30 min after injection, N = 8) for ⁶⁸Ga-AMBA and 1.6 ± 0.5%ID/g (10–20 min after injection, N = 8) for ¹⁸F-FCH, which were significantly different (p <0.001). The results in PC-3 tumour-bearing mice were comparable. Biodistribution data were in accordance with the PET results showing VCaP tumour uptake values of 9.5 ± 4.8%ID/g (N = 8) for ⁶⁸Ga-AMBA and 2.1 ± 0.4%ID/g (N = 8) for ¹⁸F-FCH. Apart from the GRPR-expressing organs, uptake in all organs was lower for ⁶⁸Ga-AMBA than for ¹⁸F-FCH.

Conclusion

Tumour uptake of ⁶⁸Ga-AMBA was higher while overall background activity was lower than observed for ¹⁸F-FCH in the same PC-bearing mice. These results suggest that peptide receptor-based targeting using the bombesin analogue AMBA is superior to metabolism-based targeting using choline for scintigraphy of PC.

[PET/CT for diagnostics and therapy stratification of lung cancer]

With the introduction of positron emission tomography (PET) and more recently the hybrid systems PET/CT, the management of cancer patients in the treatment strategy has changed tremendously. The combination of PET with multidetector CT scanning enables the integration of metabolic and high resolution morphological image information. PET/CT is nowadays an established modality for tumor detection, characterization, staging and response monitoring. The increased installation of PET/CT systems worldwide and also the increased scientific publications underline the importance of this imaging modality. PET/CT is particular the imaging modality of choice in lung cancer staging and re-staging (T, N and M staging). The possible increased success of surgery in lung cancer patients and also the expected reduction in additional invasive diagnostics lead to benefits for both the individual patient and the healthcare system. In this review article PET and PET/CT is presented for diagnostic and therapeutic stratification in lung cancer. The fundamentals of glucose metabolism, staging, tumor recurrence and therapeutic monitoring are presented.

Anti-Angiogenic/Vascular Effects of the mTOR Inhibitor Everolimus Are Not Detectable by FDG/FLT-PET

Noninvasive functional imaging of tumors can provide valuable early-response biomarkers, in particular, for targeted chemotherapy. Using various experimental tumor models, we have investigated the ability of positron emission tomography (PET) measurements of 2-deoxy-2-[(18)F]fluoro-glucose (FDG) and 3'-deoxy-3'-[(18)F]fluorothymidine (FLT) to detect response to the allosteric mammalian target of rapamycin (mTOR) inhibitor everolimus. Tumor models were declared sensitive (murine melanoma B16/BL6 and human lung H596) or relatively insensitive (human colon HCT116 and cervical KB31), according to the IC(50) values (concentration inhibiting cell growth by 50%) for inhibition of proliferation in vitro (<10 nM and >1 microM, respectively). Everolimus strongly inhibited growth of the sensitive models in vivo but also significantly inhibited growth of the insensitive models, an effect attributable to its known anti-angiogenic/vascular properties. However, although tumor FDG and FLT uptake was significantly reduced in the sensitive models, it was not affected in the insensitive models, suggesting that endothelial-directed effects could not be detected by these PET tracers. Consistent with this hypothesis, in a well-vascularized orthotopic rat mammary tumor model, other antiangiogenic agents also failed to affect FDG uptake, despite inhibiting tumor growth. In contrast, the cytotoxic patupilone, a microtubule stabilizer, blocked tumor growth, and markedly reduced FDG uptake. These results suggest that FDG/FLT-PET may not be a suitable method for early markers of response to antiangiogenic agents and mTOR inhibitors in which anti-angiogenic/vascular effects predominate because the method could provide false-negative responses. These conclusions warrant clinical testing.

Quantified tumor t1 is a generic early-response imaging biomarker for chemotherapy reflecting cell viability

PURPOSE

Identification of a generic response biomarker by comparison of chemotherapeutics with different action mechanisms on several noninvasive biomarkers in experimental tumor models.

EXPERIMENTAL DESIGN

The spin-lattice relaxation time of water protons (T(1)) was quantified using an inversion recovery-TrueFISP magnetic resonance imaging method in eight different experimental tumor models before and after treatment at several different time points with five different chemotherapeutics. Effects on T(1) were compared with other minimally invasive biomarkers including vascular parameters, apparent diffusion coefficient, and interstitial fluid pressure, and were correlated with efficacy at the endpoint and histologic parameters.

RESULTS

In all cases, successful chemotherapy significantly lowered tumor T(1) compared with vehicle and the fractional change in T(1) (DeltaT(1)) correlated with the eventual change in tumor size (range: r(2) = 0.21, P < 0.05 to r(2) = 0.73, P < 0.0001), except for models specifically resistant to that drug. In RIF-1 tumors, interstitial fluid pressure was decreased, but apparent diffusion coefficient and permeability increased in response to the microtubule stabilizer patupilone and 5-fluorouracil. Although DeltaT(1) was small (maximum of -20%), the variability was very low (5%) compared with other magnetic resonance imaging methods (24-48%). Analyses ex vivo showed unchanged necrosis, increased apoptosis, and decreased %Ki67 and total choline, but only Ki67 and choline correlated with DeltaT(1). Correlation of Ki67 and DeltaT(1) were observed in other models using patupilone, paclitaxel, a VEGF-R inhibitor, and the mammalian target of rapamycin inhibitor everolimus.

CONCLUSIONS

These results suggest that a decrease in tumor T(1) reflects hypocellularity and is a generic marker of response. The speed and robustness of the method should facilitate its use in clinical trials.

Tumor interstitial fluid pressure as an early-response marker for anticancer therapeutics

Solid tumors have a raised interstitial fluid pressure (IFP) due to high vessel permeability, low lymphatic drainage, poor perfusion, and high cell density around the blood vessels. To investigate tumor IFP as an early-response biomarker, we have tested the effect of seven anticancer chemotherapeutics including cytotoxics and targeted cytostatics in 13 experimental tumor models. IFP was recorded with the wick-in-needle method. Models were either ectopic or orthotopic and included mouse and rat syngeneic as well as human xenografts in nude mice. The mean basal IFP was between 4.4 and 15.2mm Hg; IFP was lowest in human tumor xenografts and highest in rat syngeneic models. Where measured, basal IFP correlated positively with relative tumor blood volume (rTBV) determined by dynamic contrast-enhanced magnetic resonance imaging. Most chemotherapeutics sooner (2 or 3 days) or later (6 or 7 days) lowered tumor IFP significantly, and the cytotoxic patupilone caused the greatest decrease in IFP. In rat mammary orthotopic BN472 tumors, significant drug-induced decreases in IFP and rTBV correlated positively with each other for both patupilone and the cytostatic vatalanib. In the two orthotopic models studied, early decreases in IFP were significantly (P < or = .005) correlated with late changes in tumor volume. Thus, drug-induced decreases in tumor IFP are an early marker of response to therapy, which could aid clinical development.