Investigations into Therapeutic Dose-Response / Tumor Metronomics

 
Frustrating the goal of sustained therapeutic efficacy against cancer is the shifting nature of tumor response. These shifts are a natural consequence of tumor heterogeneity. Some of these shifts can be a direct result of treatment itself, occurring because as treatment progresses, tumor subpopulations are differentially affected, altering the overall character of the tumor that faces further increments of the same regimen. In the simplest case, an acute dose can preferentially spare more resistant tumor subpopulations, which will result in a tumor that is less responsive to the same dose when given soon thereafter. But this type of resistance is highly dynamical, suggesting that if a dose schedule is spread out into smaller, uniform, regularly-spaced doses, i.e., ‘metronomically’, a better overall outcome might be attained. Early experiments by Browder et al. in mice confirmed an improved outcome, showing that even a drug to which a tumor population had become resistant under acute delivery could regain efficacy when delivered metronomically. An argument presented then for the improved efficacy was a shift in the drug target from the tumor parenchyma to the tumor stroma, specifically the supporting neovasculature. Collaterally, the milder dose increments were observed to be less toxic to the host, opening the door for higher levels of agent to be delivered safely and for benefit to improve yet further. hahnfeldt In this way, metronomic chemotherapy came to be identified as a means to gain improved potency from a drug, with minimum toxicity, through an antiangiogenic effect. Indeed, the concept has entered into the definition itself. Quoting Dr. Harold Burstein, “The definition of metronomic chemotherapy varies, but generally it refers to repetitive, low doses of chemotherapy drugs designed to minimize toxicity and target the endothelium or tumor stroma as opposed to targeting the tumor.’ Of course, the original intent in the pioneering experiments with metronomic dosing was not to gain an antiangiogenic advantage, since that property would only be discovered later as an important consequence. Nevertheless, such a working definition has pervaded the literature, drawing focus to physiologic outcomes while begging the questions of just what metronomic dosing is in terms of dose scheduling, how alternative regimens should be compared, and most importantly, how it works.

In stark contrast to the up-front, maximum tolerated dosing (MTD) strategy commonly employed in chemotherapy, metronomic dosing refers to the spreading out of a dose over time so that dose is delivered in small, regularly-spaced increments. For chemotherapy, this means dividing the dose up into small equal increments across a chemotherapeutic cycle, then repeating this pattern from one cycle to the next. Strictly speaking, no particular treatment effect or outcome is tied to the definition, although our group has uncovered some hallmark quantitative properties of the strategy stemming from sensitivity heterogeneity in the target population. In particular, for radiation or chemotherapy treatments demonstrating log-linear kill kinetics, we have established that any form of protracted dosing to an asynchronous cell population would be asymptotically more suppressive (in terms of the Malthusian ‘ultimate amplitude’ of recovered long-term exponential growth) than an up-front acute dose of the same magnitude [Hahnfeldt and Hlatky, 1996; Hahnfeldt and Hlatky, 1998]. We have followed up on this to show that, among protracted dosing schemes, uniform dosing is optimal in this regard [Hahnfeldt, Folkman, and Hlakty, 2003]. As we have shown, the effect has to do with the ongoing tendency of the heterogeneous population to ‘resensitize’ as dosing progresses. Because endothelium would be expected to be more efficient in this regard than tumor cells, we rationalized that metronomic dosing would naturally favor endothelial cell kill, thus be more antiangiogenic, than its up-front MTD counterpart.

As these studies make clear, metronomic dosing is a departure from MTD in that the goal is no longer maximizing the probability of up-front population eradication, but about optimizing long-term tumor suppression. When eradication is an unlikely event, the alternative goal of chronic, long-term tumor suppression is not only reasonable, but the de facto goal of all follow-up treatments for recurrent or metastatic disease. However, despite the diametric differences in both approach and objective, efficacies of metronomic regimens are being held to the same stringent short-term response standards as MTD protocols. This has complicated the proper evaluation of these promising new strategies.

The purpose of the research in our group is to explore the totality of features of metronomic dosing, and of dose response generally, examining essential dependencies on dose sizes and timings in the context of the dynamical tumor/host system. The belief is that, with guiding principles in hand, more constructive exploitation of dose response phenomena in treatment will be possible.
 

Metronomic Chemotherapy

The concept of metronomic chemotherapy was introduced in 20001, 2 but the general acceptance of the model in clinics has been hindered by inconsistencies in the definition of the concept. The most widely accepted definition is that it is a combination therapy which employs continuous, frequent, low doses of chemotherapeutic agents and has an angiogenic1, 3-6, stromal7 or more recently immunologic8 tumor target. Most clinicians interpret this to simply mean frequent administration of established chemotherapeutic drugs at doses below the maximal tolerated dose (MTD). This is not entirely correct.

Choice of Agent
Figure1 Dividing the traditional MTD (usually given every three weeks to allow for bone marrow recovery), into weekly doses does not necessarily result in the optimal antitumor effect. The pharmacokinetics, the mechanism of action of a drug and hormesis9, 10 must be considered. Not all traditional chemotherapetics are equipotent in the metronomic setting [Figure 1]. Picomolar doses of a tubulin inhibitor can have a very pronounced effect on endothelial cell survival11, 12, and there is a significant difference even between the different tubulin inhibitors12.

Although all chemotherapeutic agents have some anti-angiogenic effect by virtue of their non-specific cell kill, tubulin inhibitors have anti-angiogenic activity at doses well below those necessary for cell death. Low, nontoxic doses of tubulin inhibitors (vincristine, vinblastine, and taxanes) are especially efficacious in the metronomic setting, because endothelial cells are sensitive to picomolar doses of these agents13-15. This sensitivity should not be surprising in polarized endothelial cells. The orientation and integrity of endothelial cells is dependent on the maintenance of a distinctly different luminal and abluminal surfaces, and this difference in membrane polarization is maintained by a constant and precarious cytoskeletal tension16, 17. Even minimal doses of tubulin agents disrupt this tension. The target plasma concentration of tubulin agents used in metronomic setting should therefore be well below the plasma concentration usually achieved with standard doses. The standard levels (often in the low micromolar range), are 4 to 5 order of magnitude higher than those needed for endothelial cell inhibition.

Choice of Frequency
The “low” dose, how “frequent”, or which agent is continuously being renegotiated, redefined, and revised for metronomic chemotherapy. At present, most investigators simply divide the MTD of the usual agent used to treat a particular tumor into weekly doses. While such approach incorporates the historical knowledge about tumor sensitivity to the agent, it is not optimized for the sensitivity of the stroma, the target of metronomic therapy. It is also unclear why a weekly dose of many of these agents have a half-life of hours rather than days. The weekly regimen may be guided more by the fact that most chemotherapeutic agents are given intravenously — and more than weekly visits to the clinic would be unsustainable — than by optimization of schedule. A true optimization of schedule may require incorporation of plasma half-life of the metronomic agent so that a continuous exposure is achieved. For example, the half-life of vinblastine is 24 hours and optimally, the drug should be given daily. The half-life of vincristine is 19–155 hours; its excretion is 90% biliary and 10% in urine and the half-life is highly dependent on its ability to bind proteins. While it is possible to dose weekly, its inter-patient efficacy may be variable. The most optimal agent for weekly metronomic dosing may be docetaxel with a half-life of 86 hours and a very high efficacy in disrupting endothelial cell cytoskeletal integrity, migration and function.

More information is needed about pharmacokinetics of low, continuous doses before appropriate metronomic regimens are designed. Some emerging oral tubulin inhibitors such as IMC-038525, Tesetaxel (Genta®), ABT 751 (Abbott®), MPI 443803 (Myrexis®), CYT997 (Cytopia Research®) may change the scope of present clinical practice, but until they are developed for routine clinical practice, the manner in which metronomic chemotherapy can be presently applied in clinics is limited to weekly administration of docetaxel or to orally available agents such as methotrexate or cyclophosphamide. Two main guiding principles should be observed: i) A “minimally effective concentration”, rather than a “cumulative dose” should be employed; ii) No breaks in therapy should be given. If bone marrow toxicity limits the dose, lowering the dose rather than giving a break from therapy should become the practice.

Combination Metronomic Chemotherapy
Oncological practice has undergone significant changes over the last three decades. There is an increasing awareness that the same histological diagnosis may harbor very different somatic oncogenic mutations and the same tumor may have very different growth dynamic in the background of a germinal tumor suppressor gene alteration. Many tumor specific targets have been identified and numerous biologic response modifiers have been developed. Unfortunately, the introduction of these agents to clinics has been disappointing. Even in situations where a tumor responds to the therapy, the overall survival is often not improved because of increased toxicity. The oncological community has been slow to appreciate the remarkable ability of biologic response modifiers to “sensitize” to chemotherapy. While many of these agents are non-toxic in a monotherapy setting and cruise through the Phase I trials, they significantly upgrade the toxicity to standard chemotherapeutic regimens.

Metronomic dosing of chemotherapy is therefore perfectly positioned to maximize the affect of anti-angiogenic agents, immune response modifiers, proteasome inhibitors and tyrosine kinase inhibitors. Metronomic chemotherapy synergizes with novel biologic therapies while minimizing toxicity. Because neither metronomic chemotherapy alone nor biologic response modifier alone can reverse cancer progression, metronomic chemotherapy should be considered in combination with TKIs, anti-angiogenic agents or immunomodulators. While the majority of low dose, continuous chemotherapeutic agent applications will lead to stabilization or to no clinically evident effect, combination therapies can result in significant tumor regressions1, particularly in residual disease setting.

Figure2
Figure 2. Metronomic vinblastine synergizes with an anti-angiogenic agent, a VEGFR2 inhibitor DC101.
In a preclinical model of human neuroblastoma in SCID mice, neither bi-weekly vinblastine nor bi-weekly VEGFR2 inhibitor DC101 resulted in sustained tumor growth inhibition. The combination, however, led to sustained suppression of tumor growth with minimal to no toxicity. Body weight was used as a surrogate of well being, and except for a brief diarrheal episode in the group receiving combination therapy, the mice continued to gain weight. [Adapted from Klement et al, 2000]
 

The use of metronomic therapy in combination with biologic response modifiers may represent a significant change from traditional practice of oncology. As a first step, the use of Phase I–IV trials as means of early clinical testing will have to be replaced with a new clinical trial structure. For many biological agents establishing a maximum tolerated dose in a Phase I trial is not only irrelevant, but may be harmful. It is becoming increasingly more evident that the effect of many biologically active agents at high doses is often opposite to the desired effect9.

The successful use of metronomic chemotherapy in clinics will depend on our ability to change present practice of oncology. While the toxic, cytoreductive, fairly aggressive regimens may remain a necessity in the newly diagnosed disseminated and rapidly progressive disease — this may be different for early disease or for recurrence. In most cancers, achieving first line response is not difficult. Unfortunately, relapses are common and chemotherapeutic resistance hinders subsequent therapy. As we refine histological diagnosis with genomic and proteomic information about the pathways driving tumor growth and therapeutic resistance, today's clinical practice should begin incorporating this information. Patients in remission, but with a high likelihood of recurrence should be offered a maintenance therapy that employs a combination metronomic chemotherapy and a personalized choice biologic response modifier based on the genomic and proteomic analysis of their tissue.

It should be noted that successful cancer therapies, such as for example treatment of Acute Lymphocytic Leukemia in children, already employ these maintenance regimen. Similarly, in other fields of medicine, the treatment of patients with infectious diseases, hypertension, and seizures employ similarly personalized regimens. Establishing the variations in frequency, duration, and dose of metronomic chemotherapy will be critical to success.

References
1  Klement G, Baruchel S, Rak J, Man S, Clark K, Hicklin DJ, et al. Continuous low-dose therapy with vinblastine and VEGF receptor-2 antibody induces sustained tumor regression without overt toxicity. J Clin Invest. 2000 Apr;105(8):R15-24.
2  Hanahan D, Bergers G, Bergsland E. Less is more, regularly: metronomic dosing of cytotoxic drugs can target tumor angiogenesis in mice. J Clin Invest. 2000 Apr;105(8):1045-7.
3  Browder T, Butterfield CE, Kraling BM, Shi B, Marshall B, O'Reilly MS, et al. Antiangiogenic scheduling of chemotherapy improves efficacy against experimental drug-resistant cancer. Cancer Res. 2000 Apr 1;60(7):1878-86.
4  Kerbel RS, Kamen BA. The anti-angiogenic basis of metronomic chemotherapy. Nat Rev Cancer. 2004 Jun;4(6):423-36.
5  Kerbel RS, Klement G, Pritchard KI, Kamen B. Continuous low-dose anti-angiogenic/ metronomic chemotherapy: from the research laboratory into the oncology clinic. Ann Oncol. 2002 Jan;13(1):12-5.
6  Klement G, Huang P, Mayer B, Green SK, Man S, Bohlen P, et al. Differences in therapeutic indexes of combination metronomic chemotherapy and an anti-VEGFR-2 antibody in multidrug-resistant human breast cancer xenografts. Clin Cancer Res. 2002 Jan;8(1):221-32.
7  Hafner CR, Albrecht; Vogt, Thomas. New Indications for Established Drugs: Combined Tumor-Stroma-Targeted Cancer Therapy with PPAR Agonists, COX-2 Inhibitors, mTOR Antagonists and Metronomic Chemotherapy Current Cancer Drug Targets. 2005 September 2005;5(6):393-419.
8  Ghirighelli F, Menard C, Puig PE, Ladoire s, Roux S, Martin F, et al. Metronomic cyclophosphamide regimen selectively depletes CD4+CD25+ regulatory T cells and restores T and NK effector functions in end stage cancer patients. Cancer Immunol Immunother. 2007 May;56(5):641-8.
9  Reynolds AR. Potential relevance of bell-shaped and u-shaped dose-responses for the therapeutic targeting of angiogenesis in cancer. Dose Response. 2009;8(3):253-84.
10  Reynolds AR, Hart IR, Watson AR, Welti JC, Silva RG, Robinson SD, et al. Stimulation of tumor growth and angiogenesis by low concentrations of RGD-mimetic integrin inhibitors. Nat Med. 2009 Apr;15(4):392-400.
11  Ribatti D, Guidolin D, Conconi MT, Nico B, Baiguera S, Parnigotto PP, et al. Vinblastine inhibits the angiogenic response induced by adrenomedullin in vitro and in vivo. Oncogene. 2003 Sep 25;22(41):6458-61.
12  Vacca A, Ribatti D, Iurlaro M, Merchionne F, Nico B, Ria R, et al. Docetaxel versus paclitaxel for antiangiogenesis. J Hematother Stem Cell Res. 2002 Feb;11(1):103-18.
13  Wang J, Lou P, Lesniewski R, Henkin J. Paclitaxel at ultra low concentrations inhibits angiogenesis without affecting cellular microtubule assembly. Anticancer Drugs. 2003 Jan;14(1):13-9.
14  Bocci G, Nicolaou KC, Kerbel RS. Protracted low-dose effects on human endothelial cell proliferation and survival in vitro reveal a selective antiangiogenic window for various chemotherapeutic drugs. Cancer Res. 2002 Dec 1;62(23):6938-43.
15  Vacca A, Iurlaro M, Ribatti D, Minischetti M, Nico B, Ria R, et al. Antiangiogenesis is produced by nontoxic doses of vinblastine. Blood. 1999 Dec 15;94(12):4143-55.
16  Ingber DE. Tensegrity II. How structural networks influence cellular information processing networks. J Cell Sci. 2003 Apr 15;116(Pt 8):1397-408.
17  Ingber DE. Tensegrity I. Cell structure and hierarchical systems biology. J Cell Sci. 2003 Apr 1;116(Pt 7):1157-73.
  

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A strong body of work on the topic of tumor metronomics has been published by researchers currently at CCSB:

Other metronomics publications of note: