A novel retinoic acid receptor-selective retinoid, ALRT1550, has potent antitumor activity against human oral squamous carcinoma xenografts in nude mice

We have identified a novel retinoid, ALRT1550, that potently and selectively activates retinoic acid receptors (RARs). ALRT1550 binds RARs with Kd values of approximately equal to 1-4 nM, and retinoid X receptors with low affinities (Kd approximately equal to 270-556 nM). We studied the effects of ALRT1550 on cellular proliferation in squamous carcinoma cells. ALRT1550 inhibited in vitro proliferation of UMSCC-22B cells in a concentration-dependent manner with an IC50 value of 0.22 +/- 0.1 (SE) nM. 9-cis-Retinoic acid (ALRT1057), a pan agonist retinoid that activates RARs and retinoid X receptors, inhibited proliferation with an IC50 value of 81 +/- 29 nM. In vivo, as tumor xenografts in nude mice, UMSCC-22B formed well-differentiated squamous carcinomas, and oral administration (daily, 5 days/week) of ALRT1550, begun 3 days after implanting tumor cells, inhibited tumor growth by up to 89% in a dose-dependent manner over the range of 3-75 micrograms/kg. ALRT1550 (30 micrograms/kg) also inhibited growth of established tumors by 72 +/- 3% when tumors were allowed to grow to approximately equal to 100 mm3 before dosing began. In comparison, 9-cis retinoic acid at 30 mg/kg inhibited growth of established tumors by 73 +/- 5%. Interestingly, retinoids did not appear to alter tumor morphologies in UMSCC-22B tumors. Notably, ALRT1550 produced a therapeutic index of approximately equal to 17 in this model, indicating a separation between doses that inhibited tumor growth and that induced symptoms of hypervitaminosis A. In summary, ALRT1550 potently inhibits cellular proliferation in vitro and in vivo in this squamous cell carcinoma tumor model. These data support additional study of ALRT1550 for its potential for improving anticancer therapy in human clinical trials.     

Phase I study of 9-cis-retinoic acid (ALRT1057 capsules) in adults with advanced cancer

9-cis-Retinoic acid (9-cis-RA) and all-trans-RA (ATRA) are naturally occurring hormones. The nuclear receptors that mediate the effects of retinoids are the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). ATRA binds RAR with high affinity but does not bind to RXR, whereas 9-cis-RA, an isomer of ATRA, is a ligand that binds and transactivates both RARs and RXRs. The goals of this study were to determine the safety, tolerability, pharmacokinetics, and metabolic profile of 9-cis-RA in advanced cancer patients. Forty-one patients received oral 9-cis-RA (ALRT1057; Panretin capsules) at doses ranging from 5-140 mg/m2/day. Twenty-six patients were treated once daily with up to 140 mg/m2; a subsequent cohort of 15 patients were treated twice daily (b.i.d.) at 100-140 mg/m2/day (50, 60, and 70 mg/m2 b.i.d.) to evaluate a b.i.d. dosing regimen. Headache was the most frequent adverse event and was dose limiting in 3 of 41 patients. Skin toxicity was the next most common toxicity and was seen in 11 of 41 patients; it was typically mild and limited to skin dryness and erythema. Other toxicities included conjunctivitis, flushing, diarrhea, transaminitis, hypercalcemia, and asymptomatic hypertryglyceridemia. Toxicities were typically dose related, occurred primarily above 83 mg/m2/day, and were not ameliorated by b.i.d. dosing. No tumor responses were observed. The mean day 1 area under the plasma concentration-time curve and peak plasma concentration values were dose-proportional over all dose levels, whereas day 15 area under the plasma concentration-time curve and peak plasma concentration values were nonlinear above 83 mg/m2/day, suggesting that 9-cis-RA induced its own metabolism at doses equal to and above 140 mg/m2/day. 9-cis-RA is a retinoid receptor pan agonist with a more favorable pharmacokinetic and toxicity profile than that observed with previously studied retinoids and merits further investigation.     

Effects of retinoic acid (all-trans and 9-cis) on tumor progression in small-cell lung carcinoma

ON direction-selective (DS) ganglion cells were identified by electrophysiological recordings in DAPI labeled, isolated rabbit retinas. Their responses to a flashing spot were sustained. Their responses to moving stimuli were strong in the preferred direction and weak in the null direction. Injection of the recorded cells with Lucifer yellow revealed that the cells had a distinct dendritic morphology, consistent with that described previously (Buhl & Peichl, 1986; Amthor et al., 1989; Famiglietti, 1992a). When neighboring cells were injected, an extensive dendritic co-fasciculation was observed. The pattern of fasciculation restricts the possible synaptic connections of the ON DS cell.     

Comparative antitumor efficacy of docetaxel and paclitaxel in nude mice bearing human tumor xenografts that overexpress the multidrug resistance protein (MRP)

BACKGROUND: Multidrug resistance has been associated with expression of the multidrug resistance protein (MRP). Recently, MRP-expression has been detected in human tumor samples of patients with breast cancer and non-small-cell lung cancer. Since taxoids are the most active drugs in the treatment of both tumor entities, the antitumor efficacies of paclitaxel and docetaxel were compared in nude mice bearing human tumor xenografts that express MRP. MATERIALS AND METHODS: Athymic nude mice (nu/nu) bearing tumor xenografts of parental human sarcoma HT1080 or MRP-expressing HT1080/DR4 cells (as confirmed by Northern blot analysis) were treated with the maximum tolerated doses (MTD) of doxorubicin ([Dx] 10 mg/kg i.v. push), paclitaxel ([PC] 50 mg/kg three-hour i.v. infusion), or docetaxel ([DC] 40 mg/kg three-hour i.v. infusion). In vitro, the activity of doxorubicin, paclitaxel and docetaxel was evaluated by the sulphorhodamine B (SRB) assay using the pyridine analogue PAK-104P (5 microM), a potent inhibitor of MRP-function. RESULTS: At their MTDs both taxoids showed significant activity against MRP-negative HT1080 xenografts with response rates of 80% (40% CR) for PC and 100% (60% CR) for DC. In contrast, DC was significantly more active than PC in nude mice bearing doxorubicin resistant MRP-expressing HT1080/DR4 tumor xenografts (overall response rates: 100% (60% CR) for DC; 10% (0% CR) for PC; 0% for Dx). Since treatment of mice with the MTD of PC or DC yielded similar overall toxicity (maximum weight loss for HT1080: PC 8.6 +/- 2.2%; DC 7.5 +/- 2.2% and for HT1080/DR4: PC 11.6 +/- 3.0%; DC 7.6 +/- 1.8%, respectively), these results demonstrate the increase in the therapeutic index for docetaxel against MRP-expressing tumors. In vitro, HT1080/DR4 cells were 270-fold, 6.4-fold and 2.8-fold more resistant than parental cells to doxorubicin, PC and DC, respectively. Pyridine analogue PAK-104P completely restored drug sensitivity to PC and DC, while no effect of PAK-104P on parental HT1080 cells was observed. CONCLUSIONS: Both taxoids, when given at their MTDs, showed significant efficacy against parental HT1080 tumor xenografts. However, docetaxel at its MTD was significantly more active against MRP-expressing tumor xenografts than paclitaxel. Furthermore, in vitro resistance of HT1080/DR4 cells was higher for PC (6.4-fold) than for DC (2.8-fold). Since PAK-104P completely restored sensitivity to both taxoids, the observed resistance appears to be related to MRP. These data suggest, that docetaxel is not as readily transported by MRP as paclitaxel leading to an increased therapeutic ratio in MRP-expressing tumors in vivo. Therefore, docetaxel may have therapeutic advantages in the clinical treatment of MRP-expressing tumors.     

Evaluation of 9-cis retinoic acid for a new remedy of human retinoblastoma

We investigated the effect of two isomers of retinoic acid (RA), all-trans RA and 9-cis RA, on the proliferation of Y79 human retinoblastoma cells. The two isomers inhibited the cell proliferation in a concentration-dependent manner. The IC50 for this inhibition by all-trans RA and 9-cis RA was 1.50 and 0.15 microM, respectively. The inhibitory effect of 9-cis RA on Y79 cell growth was observed within 24 hr, thereafter the cell number was gradually decreased. In contrast, no inhibition by all-trans RA of Y79 cell growth was observed within 24 hr, thereafter the cell number was slightly increased. In these cases, the cell viability at 4 days after the addition of 9-cis RA and all-trans RA was more than 90% and 95%, respectively. These results indicate that the two RA inhibit the proliferation of Y79 human retinoblastoma cells without inducing the cell death and that the effect of 9-cis RA on the inhibition of Y79 cell growth is much greater than that of all-trans RA.     

Retinoic acid receptor-gamma in human epidermis preferentially traps all-trans retinoic acid as its ligand rather than 9-cis retinoic acid

Brain injury in the premature infant is an extremely important problem, in part because of the large absolute number of infants affected yearly. The 2 principal brain lesions that underlie the neurological manifestations subsequently observed in premature infants are periventricular hemorrhagic infarction and periventricular leukomalacia. The emphases of this article are the neuropathological features, pathogenesis, and potential means of prevention of these 2 lesions. Recent work suggests that the ultimate goal, prevention of the lesions, is potentially achievable. Periventricular hemorrhagic infarction may be avoidable by prevention of germinal matrix-intraventricular hemorrhage, and periventricular leukomalacia by detection of impaired cerebrovascular autoregulation, prevention of impaired cerebral blood flow, and interruption of the cascade to oligodendroglial cell death by such agents as free radical scavengers.     

The efficacy of 2',2'-difluorodeoxycytidine (gemcitabine) and vinblastine combined with interferon in nude mice xenografts of human renal cell carcinoma

Recent in vitro experiments indicated strong activity of 2',2'-difluorodeoxycytidine (dFdC, gemcitabine) in human renal cell carcinoma (RCC) cell lines and an increase of efficacy by combined application of interferon (IFN). In the present study, nude mice with xenografts from ACHN- or SN12C cells were treated by dFdC, dFdC plus IFN-alpha or vinblastine (VBL) plus IFN-alpha. ACHN-xenografts were significantly more inhibited by dFdC+/-IFN-alpha than by VBL+IFN-alpha. Complete remissions (CR) were only seen by dFdC. An additional treatment with IFN-alpha shortened the time to commencement of tumor remission and increased CR of ACHN- and SN12C-tumors (40%; 7%) compared to a treatment with dFdC alone (20%; 0). dFdC+IFN-alpha reduced the number of pulmonary metastases compared to untreated animals. Survival was significantly prolonged by dFdC+/-IFN-alpha in ACHN-mice and dFdC+IFN-alpha or VBL+IFN-alpha in SN12C mice. In conclusion, experimental data confirm dFdC as a superior drug against human RCC compared to VBL. Combined therapy with IFN-alpha increased the efficacy of dFdC in terms of tumor response in immunodeficient nude mice, thus clinical studies are strongly recommended in patients with metastatic renal cell carcinoma.     

Local hyperthermia and SR 4233 enhance the antitumor effects of radioimmunotherapy in nude mice with human colonic adenocarcinoma xenografts

Local hyperthermia and the hypoxic cytotoxin SR 4233 were administered to nude mice with 693 +/- 47 mm3 (mean +/- SE) s.c. HCT-8 human colonic adenocarcinoma xenografts in an attempt to enhance the antitumor effects of radioimmunotherapy. Biodistribution studies revealed preferential binding of NR-Lu-10, a murine monoclonal antibody, to the tumors compared with an isotype-matched control antibody, CCOO16-3.A single injection of 25 microCi 90Y-NR-Lu-10 significantly inhibited tumor growth (control versus 90Y-NR-Lu-10: P = 0.048). The administration of hyperthermia at 41.5 degrees C for 1 h immediately following the injection of 111In-labeled NR-Lu-10 up-regulated tumor-associated antigen expression and increased antibody uptake in the tumors by 73% (P = 0.001) without significantly affecting antibody uptake in normal tissues. However, the heat treatment did not produce a more homogeneous distribution of the antibodies in the tumors and did not significantly enhance the tumor growth delay produced by 90Y-NR-Lu-10 (P = 0.07). The administration of local hyperthermia at 43.0 degrees C for 1 h, on the other hand, had direct cytotoxic effects (P = 0.03) and enhanced the tumor growth delay produced by 90Y-NR-Lu-10 (P = 0.01). SR 4233 also enhanced the tumor growth delay produced by 90Y-NR-Lu-10 (P = 0.03). The greatest antitumor effects were observed when both hyperthermia at 43.0 degrees C and SR 4233 were administered in combination with 90Y-NR-Lu-10 (P = 0.002). No toxicity was produced by the local hyperthermia, and the only toxicities produced by 90Y-NR-Lu-10 and SR 4233 were neutropenia and weight loss.     

Teratogenicity of 13-cis retinoic acid and phenobarbital sodium in CF-1 mice

The teratogenicity of 13-cis-retinoic acid (RA) either administered alone or following pretreatment with phenobarbital sodium (PB), was assessed. Groups of gravid CF-1 mice were administered dosages of either 10, 100, 200, or 400 mg/kg of RA orally on either days 11, 12 or 13 of gestation, in order to determine structural alterations. In addition, separate groups of mice were orally pretreated with 80 mg/kg/day of PB on days 7 through 10 of gestation prior to the administration of RA. Skeletal alterations attributed to maternal administration of either 100, 200 or 400 mg/kg of RA on days 11-13 included delayed ossification of the limbs and supraoccipital bone, the presence of extra ribs, and various sternebral defects. Soft tissue alterations included cleft palate and dilation of the renal pelves which occurred following maternal exposure on days 11 and 12-13, respectively. Significant decreases in the incidence of cleft palate and delayed ossification of the limbs were observed in those dams administered RA on days 11 or 12, respectively, following prior treatment with PB. These data suggest that administration of PB, a prototypical hepatic microsomal enzyme inducer, may partially antagonize the teratogenicity of RA.     

Gene expression and neuroblastoma cell differentiation in response to retinoic acid: differential effects of 9-cis and all-trans retinoic acid

Retinoic acid has considerable potential for the chemoprevention and chemotherapy of cancer. Neuroblastoma cells differentiate in response to retinoic acid in vitro, an observation that has led to clinical trials using either the 13-cis or all-trans isomers of retinoic acid. We review the effects of retinoic acid on neuroblastoma, and the potential involvement of nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). 9-cis retinoic acid is a ligand for RXRs, and we review recent data on the differential effects of 9-cis and all-trans retinoic acid on neuroblastoma differentiation and proliferation in vitro, and possible mechanisms of action via hetero- and homodimers of RARs and RXRs. Although there is uncertainty whether or not 9-cis retinoic acid produces its biological effects primarily via RXR homodimers, in vitro data suggest that this isomer of retinoic acid or stable analogues may have considerable potential for the treatment of resistant, disseminated neuroblastoma.     

A phase II pilot trial of 13-cis retinoic acid and interferon-alpha in patients with advanced pancreatic carcinoma

BACKGROUND: Advanced unresectable pancreatic adenocarcinoma has a dismal prognosis. The authors previously have shown that retinoic acid (RA) and interferon-alpha (IFN-alpha) inhibit growth and induce differentiation in human pancreatic carcinoma cells in vitro and in vivo. The purpose of this trial was to examine the feasibility and tolerability of a combination therapy of 13-cis RA and IFN-alpha in patients with advanced unresectable pancreatic carcinoma. METHODS: Twenty-two patients (median age, 62 years) with histologically confirmed, unresectable pancreatic adenocarcinoma classified as International Union Against Cancer Stage III (5 patients) or IV (17 patients) were included. Patients received 1 mg/kg body weight 13-cis RA orally and 6 million IU IFN-alpha subcutaneously daily. Restaging by ultrasound, computed tomography scan, and chest X-ray was performed every 2 months. RESULTS: No complete remission and 1 partial remission (PR) (4.5%) were observed. Fourteen patients (63.6%) demonstrated stable disease with a median duration of 5.0 months (range, 2.3-17.7+ months). Toxicity mainly was related to IFN-alpha and predominantly was hematologic (no toxicity was World Health Organization [WHO] Grade 4 and 13.6% were WHO Grade 3). Nonhematologic toxicities did not exceed Grade 2 (skin and oral mucosa) and mainly were related to 13-cis RA. The median survival of the patients with Stage III disease was 8.7 months (range, 6.8-23.9+ months) and was 7.4 months for patients with Stage IV disease (range, 0.9-19.2+ months), resulting in a median overall survival of 7.7 months (range, 0.9-23.9+ months). CONCLUSIONS: Combination therapy with 13-cis RA and IFN-alpha is feasible and well tolerated in patients with advanced pancreatic carcinoma. Based on the median survival rates observed in this study this combination should be investigated further in Phase III trials.     

Influence of chemotherapy on FDG uptake by human cancer xenografts in nude mice

This study evaluated the use of PET with 18F-2-deoxy-2-fluoro-D-glucose (18F-FDG) for monitoring chemotherapy effects, using a human cancer xenograft (poorly differentiated human gastric cancer) in vivo model. METHODS: Tumor 18F-FDG uptakes and sizes were measured after administrating mitomycin (MMC), cisplatin (CDDP) and adriamycin (ADR) to xenograft-bearing nude mice and compared with 18F-FDG tumor uptake and tumor size in a non-therapy group. The correlation between the uptake and size was also assessed. RESULTS: The largest reduction in tumor size after chemotherapy occurred in the MMC administered group, followed by the CDDP case, with no reduction in the ADR group as compared to the controls. Fluorine-18-FDG tumor uptake after chemotherapy was also decreased in the MMC and CDDP groups, in that order, but not in the ADR case. With MMC and CDDP, size reduction became significant on Days 8 or 11, whereas 18F-FDG tumor uptake had already been decreased on Days 3 or 7. CONCLUSION: Fluorine-18-FDG uptake decreases in parallel to the efficacy of anticancer agents and correlates with subsequent morphologic changes. We conclude that 18F-FDG PET tumor images are indeed useful for monitoring the effects of cancer chemotherapy.     

Expression of laminin in renal-cell carcinomas, renal-cell carcinoma cell lines and xenografts in nude mice

We studied the expression of laminin (Ln) chains (alpha1-alpha3, beta1-beta3, gamma1) in human renal-cell carcinomas (RCC), papillary renal neoplasms (PRN) and oncocytomas, in RCC cell lines and their xenografts. In RCCs the basement membranes (BM) showed immunoreactivity for chains of Ln-1 (alpha1-beta1-gamma1). Only in well-differentiated RCCs could vessel BMs be distinguished from those of carcinoma cell islets. RCCs and oncocytomas also exhibited an abundant immunoreactivity for Ln beta2 chain in both vessel and tumor cell BMs, while Ln alpha2 chain was not seen in any renal tumors. In distinction from RCCs, PRNs presented a strong BM immunoreactivity for Ln alpha3 and beta3 chains and for Ln-5, as well as lack of Ln beta2 chain. A more variable reactivity for Ln-5 was seen in oncocytomas. As PRNs and oncocytomas have been suggested to originate from collecting ducts, it is notable that in normal human kidney, we could detect immunoreactivity for Ln-5 and its chains only in BM of the tubules of the loop of Henle. In immunoprecipitation experiments, an abundant production of Ln-1, but not of Ln-5, was seen in cultured RCC cells, while in xenografts of the same cells BM-confined immunoreactivity for both Ln-1 and Ln-5 was seen. Ln beta2 chain was produced by 2 of the 4 RCC cell lines in culture but was found only in 1 of the xenografted tumors.     

Antiretroviral efficacy and pharmacokinetics of oral bis(isopropyloxycarbonyloxymethyl)-9-(2-phosphonylmethoxypropyl)adenine in mice

To overcome the low oral bioavailability of the highly potent and selective antiretroviral agent (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA), a new lipophilic ester derivative, i.e., the bis(isopropyloxycarbonyloxymethyl)-ester [bis(POC)-PMPA], was prepared. The usefulness of bis(POC)-PMPA as an oral prodrug for PMPA was investigated in the intestinal mucosa Caco-2 cell monolayer model. The total transport of bis(POC)-PMPA was 2.7%, whereas it was less than 0.1% for PMPA. Bis(POC)-PMPA was considerably metabolized inside the epithelial cells, since the majority of the compound was recovered after transport in the form of the monoester metabolite [mono(POC)-PMPA]. In contrast, bis(POC)-PMPA was relatively resistant to degradation at the luminal side of the Caco-2 cells. Pharmacokinetic studies with mice showed that the oral bioavailability of bis(POC)-PMPA (calculated from the curves of the concentration of free PMPA in plasma) was 20%. Neither bis(POC)-PMPA nor mono(POC)-PMPA could be recovered in plasma, suggesting the efficient release of the active drug PMPA after oral administration of bis(POC)-PMPA. Severe combined immunodeficient (SCID) mice infected with Moloney murine sarcoma virus (MSV) and treated orally with bis(POC)-PMPA for 5 or 10 days (dosages, 50, 100, or 200 mg of PMPA equivalent per kg of body weight per day) showed a significant delay in MSV-induced tumor appearance and tumor-associated death. The antiviral efficacy of oral bis(POC)-PMPA was related to the dosage and treatment period and was not significantly different from that of subcutaneous PMPA given at an equivalent dose. The favorable pharmacokinetic profile, marked antiviral efficacy, and low toxicity make bis(POC)-PMPA an attractive oral prodrug of PMPA that should be further pursued in clinical studies with patients infected with human immunodeficiency virus or hepatitis B virus.     

Effect of dietary supplementation with gamma-linolenic acid on the growth of a human lung carcinoma implanted in nude mice

We have studied the effect of a gamma-linolenic acid (18:3 n-6, GLA)-supplemented diet on the growth of a human lung mucoepidermoid carcinoma (HLMC) implanted in athymic mice and on its uptake of human low density lipoproteins labeled with 99mTc (99mTc-LDL). Mice bearing the HLMC were divided into two experimental groups. One of them was administered a control diet (C diet) and the other one was given a diet supplemented with 25 mg GLA/g pellet (GLA diet) for three weeks (Table 1). A tumor growth inhibition with the GLA diet was evident at the second week of treatment, and a marked inhibition (56%) was reached at the end of the third week (Fig. 1). The GLA diet produced some changes in the total fatty acid composition of tumor, plasma and liver of host mice: GLA and arachidonic acid (20:4 n-6, AA) induced significant increases, whereas oleic (18:1 n-9, OA) and linoleic acids (18:2 n-6, LA) were decreased (Table 2). Tumors of those animals fed both diets were labeled by 99mTc-LDL, and no difference was observed in the ratio of tumor/liver and tumor/kidney uptake of host animal (Table 3). Results obtained using this experimental model suggest that the inhibitory effect of GLA on tumor growth is not related to the LDL tumor uptake.     

Preclinical evaluation of 111In-labeled B3 monoclonal antibody: biodistribution and imaging studies in nude mice bearing human epidermoid carcinoma xenografts

Biodistribution and imaging characteristics of monoclonal antibody B3 were evaluated in nude mice bearing A431 human epidermoid carcinoma xenografts. B3 is a murine IgG1k, recently isolated, reacting with a carbohydrate epitope abundantly and uniformly expressed by most carcinomas. B3 was conjugated to a new backbone-substituted derivative of diethylenetriaminepentaacetic acid, 2-(p-isothiocyanato benzyl)-cyclohexyl-diethylenetriaminepentaacetic acid, and labeled with 111In. Tumor-bearing mice were given i.v. injections of approximately 5 microCi of either 111In-B3 or 111In-MOPC-21, an isotype-matched control, and sacrificed in groups of five at 6 h and daily up to 168 h. Imaging was performed at 24, 72, and 144 h. Significant differences were observed in tumor uptake at all time points with peak values at 48 h (25 +/- 5.2% versus 6.3 +/- 0.4% of the injected dose/g tissue) (mean +/- SD) for 111In-B3 and 111In-MOPC-21, respectively (P < 0.001). All tumor to organ ratios increased with time for 111In-B3. In particular, tumor:liver ratios rose from 3.2 +/- 0.6 at 24 h to 6.3 +/- 1.2 at 168 h. Imaging results showed selective and progressive accumulation of 111In-B3 at the tumor site, whereas 111In-MOPC-21 did not show specific localization. In summary, 111In-labeled B3 demonstrated good and specific tumor targeting, which warrants its future clinical evaluation.     

Delayed vascular changes after antiangiogenic therapy with antivascular endothelial growth factor antibodies in human glioma xenografts in nude mice

OBJECTIVE: The purpose of this study was to examine the delayed effects of antivascular endothelial growth factor treatment on tumor growth and vascularity in a subcutaneous mouse tumor model of human glioblastoma. METHODS: Antivascular endothelial growth factor antibody treatment was administered for a period of 6 weeks, to suppress tumor growth. To detect late vascular effects, tumor vascular parameters for treated tumors and control tumors were analyzed 4 weeks thereafter. By that time, tumors had grown to adequate sizes (diameter, 8-10 mm) for comparison with untreated control tumors. Vascular parameters were quantified by using an image-analysis system. RESULTS: Vascular density was significantly lower in antivascular endothelial growth factor antibody-treated tumors, compared with control tumors of similar size. The vascular architecture of treated tumors was also distinctly different, compared with control tumors, showing larger but sparser vessel structures. CONCLUSION: These findings suggest that antiangiogenic therapy may have a prolonged effect on the vascular architecture of certain tumors, resulting in enduring changes in the tumor vessels. Because tumor vasculature plays an important role in the sensitivity to various treatment modalities, these changes are likely to influence the responses of these tumors to further therapy.