New Directions in the Study and Treatment of Metastatic Cancer

Abstract

Traditional cancer therapy has relied on a strictly cytotoxic approach that views non-metastatic and metastatic tumor cells as identical in terms of molecular biology and sensitivity to therapeutic intervention. Mounting evidence suggests that, in fact, non-metastatic and metastatic tumor cells differ in key characteristics that could explain the capacity of the metastatic cells to not only escape the primary organ but also to survive while in the circulation and to colonize a distant organ. Here, we lay out a framework for a new multi-pronged therapeutic approach. This approach involves modifying the local microenvironment of the primary tumor to inhibit the formation and release of metastatic cells; normalizing the microenvironment of the metastatic organ to limit the capacity of metastatic tumor cells to invade and colonize the organ; remediating the immune response to tumor neoantigens; and targeting metastatic tumor cells on a systemic level by restoring critical and unique aspects of the cell's phenotype, such as anchorage dependence. Given the limited progress against metastatic cancer using traditional therapeutic strategies, the outlined paradigm could provide a more rational alternative to patients with metastatic cancer.

Keywords: cancer microenvironment; cancer stabilization; cancer therapy; metastasis; targeted therapy.

Figure 1

Clinical evidence that vascular normalization through a single infusion of the VEGF-specific antibody bevacizumab is effective in rectal carcinoma patients. “Parameters were obtained pretreatment and after one bevacizumab infusion. (A−C) Blood perfusion (A), blood volume (B), and permeability–surface area product [PS (C)]. Significant decreases after treatment are indicated by solid lines (P < 0.05 by t-test). Blood flow and blood volume decreased significantly in four of the patients. (D) Microvascular density. All patients showed significant decreases after treatment (P < 0.05 by t-test). (E) Fraction of vessels with pericyte coverage. The difference in the fraction of vessels positive for alpha-smooth muscle actin (alpha-SMA) in patient 2 was identified as an outlier by the Extreme Studentized Deviate test. Paired t-test analyses of the mean values that included and excluded the data of patient 2 had P < 0.09 and 0.001, respectively. (F) Mean tumor IFP decreased significantly after bevacizumab (P < 0.01 by paired t-test). (G) Tumor FDG uptake before treatment, on day 12 and presurgery (day 93), normalized for muscle values. On day 12 after bevacizumab treatment, a 40% decrease was observed in patient 3, and no change in the other patients. Lower levels were found in all patients before surgery except for patient 5, who had low levels throughout the treatment. In comparison to pretreatment and day 12 values, the median standard uptake value was significantly lower on day 93 (P < 0.01). (H) Circulating progenitor/stem cells (AC133+; left) and viable CECs (right) in peripheral blood. Samples were run to acquire 50,000 events in the mononuclear/lymphocyte gate. For both cell populations, bevacizumab induced a significant decrease in mean values (P < 0.05 by Wilcoxon signed-rank test). Key in (B) applies to (A,C,G−I).” Reprinted from Willett et al. (29) with kind permission by Nature Publishing Group.

Figure 2

Preclinical evidence that restoration of normoxia through treatment with oxygen-loaded microbubbles (O₂MB-RB) is effective in a murine model of pancreatic cancer. Ectopic human xenograft BxPC-3 tumors were established in the rear dorsum of SCID mice. Plot of (A) % change in tumor volume and (B) average body weight for mice treated with (i) no treatment (open diamonds), (ii) ultrasound only (filled diamonds), (iii) gemcitabine (open triangles), (iv) O₂MB-RB/O₂MB-5FU mix − US (open circles), (v) O₂MB-RB + US (filled squares), (vi) O₂MB-RB/O₂MB-5FU mix + US (filled circles). Not shown for ease of illustration are treatments with 5-FU alone, O₂MB-RB − US, O₂MB-5FU + US, O₂MB-5FU − US. The RB, 5-FU and gemcitabine concentrations were kept constant in each case at 0.184 mg/kg (90.8 µM), 0.115 mg/kg (440 µM), and 0.264 mg/kg (440 µM), respectively. Ultrasound treatment was delivered for 30 s at frequency of 1 MHz, an ultrasound power density of 3.0 Wcm⁻² and a duty cycle of 50%, pulse frequency = 100 Hz. Error bars represent ± SE where n = 4. *P < 0.05, **P < 0.01, and ***P < 0.001 for (vi) compared to (i) and ^ΔP < 0.05, ^ΔΔP < 0.01, and ^ΔΔΔP < 0.001 for (vi) compared to (v). “These results reveal a dramatic reduction in tumor volume for mice treated with the combined SDT/antimetabolite therapy compared to either gemcitabine or 5-FU treatment alone.” Reprinted from Mcewan et al. (78) with kind permission by Elsevier Publishing Group.

Figure 3

Preclinical evidence that increases in intracellular levels of the second messenger, cAMP, lead to activation of protein kinase A and cause mesenchymal human mammary epithelial cells to revert to their epithelial state, resulting in inhibition of tumor-initiating and metastatic potential. The authors “transplanted at limiting dilutions HMLE-Ras immortalized human mammary epithelial cells and their spontaneously arising mesenchymal derivatives, termed NAMEC8 (N8-Ras) cells, as well as N8-CTx-Ras cells that were mesenchymal-to-epithelial transition (MET) reverted by treatment with cholera toxin (CTx). Cells were implanted into the mammary fat pads of NOD/SCID mice. (A) As anticipated, the frequency of tumor-initiating cells in the N8-Ras cells was far greater than in the HMLE-Ras cell population, in this case, 100-fold higher. Significantly, the N8-CTx-Ras cells were as inefficient at tumor-initiation as the HMLE-Ras cells. (B) The primary tumors that arose upon orthotopic mammary stromal fat pad implantation of N8-Ras tumors spawned 20–30 micrometastases in the lungs by 12 weeks following implantation. This property was completely lost upon induction of an MET by CTx treatment prior to transplantation.” Reprinted from Pattabiraman et al. (90) with kind permission by the American Association for the Advancement of Science.

Figure 4

Preclinical evidence in a murine model of melanoma that lipid signaling is an important player in metastasis, which could be potentially targeted for therapy. The authors performed a genome-wide in vivo screen of 810 mutant mouse lines, which resulted in the identification of host genes that regulate metastatic colonization in the lung after injection of B16 melanoma cells. Their screen identified 23 hits including the sphingosine-1-phosphate (S1P) transporter Spns2. In the studies shown here, the authors manipulated the S1P axis pharmacologically by inhibiting S1P lyase, which degrades S1P, using 4′-deoxypyridoxine (DOP), a compound previously shown to increase lymphoid tissue S1P levels and induce a circulating lymphopenia. The studies shown here demonstrated that deletion of Spns2 by DOP treatment created a circulating lymphopenia with increased numbers of natural killer cells and effector T cells in the lung, which effectively prevented lung colonization. (A,C) Number of leukocytes and T cell subsets present in the lungs of B16-F10-dosed glucose- or DOP-treated wild-type male mice presented as the percentages of viable CD45⁺ lung leukocytes. (B,D) Number of leukocytes and T cell subsets presented as the percentages of viable parent CD4⁺ or CD8⁺ T cells. (E) Experimental metastasis assay in B16-F10 dosed glucose- or DOP-treated wild-type female mice. Reprinted from van der Weyden et al. (104) with kind permission by the Nature Publishing Group.

Figure 5

Clinical evidence that anti-osteoclastic therapy can be effective in patients with castration-resistant prostate cancer. “Time to first and subsequent on-study skeletal-related events in prostate cancer patients treated with denosumab vs. zoledronic acid.” Reprinted from Fizazi et al. (115) with kind permission by Elsevier.

Figure 6

Clinical evidence that treatment with the immune checkpoint inhibitor, nivolumab is effective against metastatic mucosal melanoma. Systemic clinical response to nivolumab treatment. Computed tomography images from baseline (May 2013), 2 months into treatment and approximately 2 years later. Yellow arrows or circles indicate metastatic foci and their complete disappearance by March 2015. The green arrow depicts the level of LDH at the selected time points. LDH lactate dehydrogenase. Reprinted from Ascierto et al. (126) with kind permission by Springer Berlin Heidelberg.

Figure 7

Preclinical evidence that metastatic-cell targeted therapy is effective in a murine model of metastatic breast cancer. “Metastatic burden and survival of mice orthotopically implanted with human breast cancer cells and treated with the miR-10b inhibitor MN-anti-miR-10b and low-dose doxorubicin (Dox). (A) Representative bioluminescence images of metastatic burden showing complete regression of metastases in animals treated with MN-anti-miR-10b and doxorubicin. (B) Quantitative analysis of metastatic burden from all treatment groups, indicating complete regression of metastatic burden in the lymph nodes of experimental animals treated with MN-anti-miR-10b and doxorubicin after just four weekly treatments. Background counts are derived from non-tumor-bearing animals. (C) Ex vivo BLI showing the absence of detectable lymph node or lung metastases in mice treated with MN-anti-miR-10b and doxorubicin. In animals treated with MN-anti-miR-10b alone, there were lymph nodes but not lung metastases. In all other groups, there were both lymph node and lung metastases. (D) Animal weight. The groups treated with MN-anti-miR-10b with or without doxorubicin continued to gain weight throughout the time course of the study. (E) Mortality. Only in the group of animals treated with MN-anti-miR-10b and doxorubicin, there was no mortality from carcinoma. Data, average ± SEM; within-subjects ANOVA: P < 0.05. PBS, n = 2; MN-scr-miR, n = 6; MN-scr-miR + Dox, n = 10; MN-anti-miR-10b, n = 7; MN-anti-miR-10b + Dox, n = 10.” Reprinted from Yoo et al. (134) with kind permission by AACR.