Although mutant RAS has proved to be a challenging therapeutic target, recent success with tipifarnib, a biologically active drug known as a farnesyltransferase inhibitor, brings promise for treating solid tumors and hematological malignancies.
Alan L. Ho, MD, PhD
For more than 20 years, researchers have known that the RAS pathway is involved in a wide variety of cancer types. RAS proteins normally switch between an active state, which is bound by guanosine triphosphate, and an inactive state, which is bound by guanosine diphosphate, to regulate cell-cycle progression. In cancer, the mutant RAS gene becomes locked in the active state, causing uncontrolled cell proliferation.1 Such mutations are found in 30% of all neoplasms, with a higher prevalence in colon cancer (~50%) and pancreatic cancer (~90%).2
Unfortunately, mutant RAS has proved to be a challenging therapeutic target. After early attempts at direct targeting were unsuccessful, subsequent research with indirect targeting led to predominantly disappointing results. No drugs are currently approved that directly target RAS activity.1,3
Despite this rocky terrain, recent success with tipifarnib, a biologically active drug known as a farnesyltransferase inhibitor (FTI), brings promise for treating solid tumors and hematological malignancies. In time, FTIs may be utilized in a variety of cancer types and other diseases as associated pathways become better defined.Farnesyltransferase (FTase) is an enzyme that plays a key role in RAS posttranslational processing (Figure).1,4 Specifically, FTase is responsible for farnesylation, a type of prenylation, in which a hydrophobic group is added to the C-terminal CAAX motif of a RAS protein. Prenylation allows for RAS membrane binding and subsequent downstream signaling; without it, mutant RAS becomes inert, thereby halting uncontrolled cell proliferation.5
Although targeting FTase initially appeared to be a logical way to stop RAS membrane binding, a major obstacle lies in enzymatic redundancy. Researchers have found that RAS prenylation can also be achieved by geranylgeranyltransferase, which means that blocking. FTase does not necessarily stop RAS membrane localization. This scenario likely explains the disappointing results of earlier FTI trials.2 Recent research, however, exploits the fact that not all RAS isoforms are so dynamic.6
The RAS family isoforms are KRAS, NRAS, and HRAS. Of these 3, HRAS exclusively relies upon FTase for prenylation, which means that FTIs are still effective in HRAS-driven cancer types.3 Ongoing research into this subclass of RAS proteins is yielding promising results. A study by Alan L. Ho, MD, PhD, a medical oncologist and the Geoffrey Beene Junior Faculty Chair at Memorial Sloan Kettering Cancer Center, is investigating the efficacy of tipifarnib, a first-in-class, highly selective FTI that competitively binds to the CAAX motif of FTase. Treatment with tipifarnib has produced partial responses in 4 of 6 patients with HRAS-mutant head and neck squamous cell carcinoma (HNSCC).7
“This evidence is the first to really demonstrate that mutant HRAS is a target in cancer with FTIs,” Ho said in an interview. “The activity we’ve seen [with tipifarnib] is rapid and durable and has translated into clinical benefit in a number of different ways.”8
Treated patients had HNSCC and an HRAS mutation, with no available curative treatments. Tipifarnib was administered orally at 900 mg twice daily during alternating weeks in a 28-day cycle. Of the 4 responding patients, 2 responded for over 1 year. The patients who did not respond maintained stable disease during the trial, and tumor size decreased in all patients.
Ho said that the objective response rate of 67% (95% CI, 22%-95%) “is a remarkable response rate for a previously treated patient population.” The phase II trial was initiated in light of previous research surrounding HRAS susceptibility and anecdotal evidence that tipifarnib was effective in some patients as a single agent. Tipifarnib has been used in over 70 studies that included more than 5000 patients, and is relatively well tolerated, with less than 25% of patients discontinuing treatment due to adverse events (AEs).
Among 27 patients treated across 3 cohorts in the study, grade ≥3 treatment-emergent AEs included myelosuppression (neutropenia, 31%; anemia, 19%; and thrombocytopenia, 15%), gastrointestinal disturbances (15%), and increased creatinine (11%).
The study is currently ongoing with 2 other patient cohorts, including a group of patients with HRAS-mutant thyroid carcinoma and a group of patients with squamous cell carcinoma not of the head and neck.7Previous studies’ results have shown that tipifarnib can generate major responses in some patients with myelodysplastic syndromes (MDS) or acute myeloid leukemia (AML), but the overall activity and molecular mechanisms behind these responses has remained unclear. Although this mystery has thus far precluded drug registration, recent research suggests that tipifarnib may target the CXCL12/CXCR4 pathway.9
The CXCL12 (stromal cell-derived factor-1)/ CXCR4 (CXC receptor 4) axis plays a part in a variety of neoplastic events, including metastasis, survival, and angiogenesis. As a homeostatic chemokine, CXCL12 controls secondary lymphoid tissue architecture and hematopoietic cell trafficking. CXCR4 activity is thought to involve the RAS-activated signaling pathway, although exact mechanisms are unknown. CXCR4 is broadly expressed on hematopoietic cells such as B lymphocytes, T lymphocytes, CD34-positive hematopoietic stem cells (HSCs), macrophages, monocytes, eosinophils, and neutrophils. Further expression of CXCR4 can be found on colon, lung, heart, brain, liver, kidney, epithelial, endothelial, and some progenitor cells. Functional CXCR4 is expressed on several types of tissue-committed stem cells and embryonic pluripotent stem cells, allowing them to invade and/or migrate along CXCL12 gradients.10
Previous research has found that CXCL12/ CXCR4 signaling causes the bone marrow to retain neoplastic cells, which protects them from apoptosis. Findings from clinical trials in patients with multiple myeloma and non- Hodgkin lymphoma showed that treatment with plerixafor, a small molecule inhibitor of CXCR4, prompted cellular egress from bone marrow, thereby increasing collection yield for later HSC transplant. Additionally, a mouse model of acute promyelocytic leukemia revealed that treatment with a CXCR4 antagonist improved the efficacy of cytarabine, as bone marrow protection was lost when neoplastic cells were released into circulation. These findings affirm that increased CXCL12/CXCR4 causes cell retention in the bone marrow, making it an attractive target in bone marrow neoplasia.
In a 2014 study involving patients with AML, treatment with tipifarnib at 300 mg twice daily for 3 weeks led to response rates of up to 20%.11 However, patient-specific responses could not be correlated with blast karyotype, clinical features, FTase inhibition, or RAS mutation status. With regard to this finding, the researchers noted that a reliable predictor of response to tipifarnib was still lacking.
Fortunately, Antonio Gualberto, MD, PhD, and his team at Kura Oncology, Inc. may be closing in on an answer. In recent findings presented at the 2017 American Society of Hematology Annual Meeting, investigators showed that tipifarnib may target the CXCL12/CXCR4 pathway.9 In patients with AML and MDS, tipifarnib was most effective when high levels of CXCL12 were found in the bone marrow. The researchers concluded that a high level of CXCR4 compared with a low level of the antagonistic receptor CXCR2 may serve as a reliable biomarker for tipifarnib in bone marrow neoplasia.
In a group of 58 patients with relapsed or refractory AML who were treated with tipifarnib, the quintile expressing the highest CXCR4/CXCR2 ratios achieved progression-free survival (PFS) times nearly double those of all other patients (57 days vs 29 days; P = .026). When tipifarnib was administered to another cohort of 15 patients with chronic myelomonocytic leukemia, the tertile with the highest CXCR4/CXCR2 ratios achieved a PFS of 280 days compared with 84 days for those with lower levels (P = .015).
The researchers noted that tipifarnib has a safety profile at least as favorable as best supportive care including hydroxyurea. With older and more frail patients with AML, tipifarnib could be a more attractive option than chemotherapy, particularly when a high CXCR4/CXCR2 ratio is detected.Although combination therapies have yielded mixed results, FTIs may increase sensitivity to chemotherapeutics or radiation with appropriate timing, particularly in HRAS-mutant cancer types.5 In light of the recent successes with tipifarnib in HRAS-mutant HNSCC, more combination studies may be forthcoming. Kura Oncology, a biopharmaceutical company headquartered in San Diego, California, has 4 ongoing clinical trials investigating tipifarnib in HNSCC and myeloid malignancies (Table).
Following a meeting with the FDA, the company said it plans to initiate a registration- directed phase II trial in patients with HRAS-mutant HNSCC in the second half of 2018. The single-arm study, to be called AIM-HN, would seek to enroll at least 59 patients with recurrent or metastatic disease.12
At press time, tipifarnib remains the only FTI undergoing clinical trials for use in cancer treatment, while previously investigated FTIs BMS-214662, CP-609,754, and AZD3409 remain dormant, according to a search of the ClinicalTrials.gov website. Outside of the cancer arena, however, research with other FTIs continues.
Of note, lonafarnib is undergoing clinical trials for Hutchinson-Gilford Progeria Syndrome (HGPS), a terminal illness that causes premature aging. In patients with HGPS, progerin is the protein thought to be responsible for blocking normal cell function, and as farnesylation is required for progerin activity, lonafarnib could be the first therapeutic drug for this rare disease. Completed trials have recorded increased survival times in treated patients.13
To date, more than 50 proteins are known to undergo posttranslational farnesylation, and additional activity may be elucidated in the future. Further research is needed to better understand associated pathways. As early trials using FTIs to indirectly target the RAS pathway are found, blocking farnesylation is inadequate as a therapeutic strategy for all tumors with RAS mutations.
However, ongoing research shows that HRAS-mutant varieties appear susceptible to tipifarnib due to a lack of redundant enzymes. Along the same optimistic lines, research into the CXCL12/CXCR4 pathway is defining which patients with hematological malignancies are likely to respond to FTI therapy and illuminating associations with the RAS pathway. As research clarifies the complex network of pathways that drive neoplasia and other diseases, more patient-specific therapies may be on the horizon.
“Analysis of CXCR4 and CXCR2 expression in bone marrow aspirates of mononuclear cells revealed an association between the ratio of CXCR4 to CXCR2 and the clinical activity of tipifarnib,” investigators reported. This correlation “was consistent across endpoints, clinical settings, and indications,” they added. Ongoing phase II clinical trials aim to elucidate these findings by researching upstream and downstream farnesylated targets in the CXCL12/CXCR4 pathway.