Therapeutic Strategies for Ovarian Cancer

Over the last decades, biomarkers have become an extremely important tool in the care of patients with ovarian cancer. Biomarkers are capable of differentiating benign versus malignant masses, detecting the disease at an earlier stage, and monitoring the cancer’s response to treatment and its recurrence¹.

Patients with epithelial ovarian cancer (EOC) have a 5-year survival rate for all stages estimated at 45.6%², but when effective early stage detection is possible (in roughly 20% of cases), the survival rate reaches 70%³. It is worth noting that roughly three-quarters of ovarian cancers are diagnosed when the disease has progressed to advanced-stages (stage III or IV), and has spread to peritoneal surfaces, or other organs⁴. This trend explains the high mortality rate of advanced stage disease at initial diagnosis⁵. Importantly, as the disease progresses, treating and managing patients becomes more challenging³.

For example, when the disease is detected at stage I (where the cancer is limited to the ovaries), 90% of patients can be cured with surgery, chemotherapy and radiotherapy. Similarly, when the cancer has spread to the pelvis in stage II, 70% of the patients can still be cured. In more advanced stages where the cancer has spread throughout the abdomen or beyond, the percentage of curable patients drops to 20%¹. These differences in survival rates may also reflect the fact that currently available treatment strategies are much more successful in early stages, when the cancer has not yet spread⁷.

As previously mentioned, current treatment strategies combine debulking surgeries, drug treatment and radiation therapy. Depending on the type of ovarian cancer, treatment options may include: chemotherapy, hormone therapy, and targeted therapy.

Chemotherapy: Administered intravenously (IV), intraperitonially (IP), or by a combination of IV/IP, chemotherapeutic agents are selected based on the stage of OC. The most frequently used are platinum-based drugs (cis-platin and the less toxic carboplatin), and drugs in the Taxane family (Paclitaxel and Docetaxel)³. Specifically, one common IV chemotherapy regimen is the combination of Carboplatin (AUC 6-7) with Paclitaxel (175mg/ m2) for three to six cycles⁸.

Hormone Therapy: Including hormones or hormone-blocking drugs, this systemic therapy is most often used to treat ovarian stromal cancers, but rarely used for EOC⁹. Therapy includes luteinizing-hormone-releasing hormone (LHRH) agonists that lower estrogen levels in premenopausal women, tamoxifen (often used to treat breast cancer and in ovarian stromal cancer but rarely used in advanced EOC), which also has anti-estrogen activity, and aromatase inhibitors that help reduce estrogen levels by inhibiting the enzyme aromatase that turns other hormones into estrogen in post-menopausal women⁹.

Targeted Therapy: Drugs that identify and attack cancer cells specifically while doing little damage to normal cells. Targeted therapies change how cancer cells grow, divide, repair themselves, or interact with other cells¹⁰. For example, angiogenesis (the physiological process through which new blood vessels form from pre-existing vessels) plays a key role in the initiation and progression of ovarian carcinogenesis¹¹. Bevacizumab (Avastin), an angiogenesis inhibitor, acts by interfering with a protein called VEGF (that signals and promotes the formation of new blood vessels) and slows or stops cancer growth. Bevacizumab has been shown to increase the overall survival of high risk patients in a phase 3 randomized study¹². Another group of targeted therapy drugs include (poly (ADP)-ribose polymerase) inhibitors (PARPi), that may help prevent cancer cells from repairing their damaged DNA, thus causing them to die. Specifically, the use of PARP inhibitors for the treatment of cancers with defects in the BRCA1 (BReast CAncer gene 1) or BRCA2 tumor suppressor proteins (which are also involved in the repair of DNA damage), appears to be a promising approach. PARPis are specifically lethal for cancer cells with homologous recombination (HR) deficiency, which characterize 50% of HGSCs^13,14. However, multiple resistance mechanisms to PARPi have also been identified^11,15.

Despite targeted therapy’s (including Bevacizumab and PARP inhibitors) strong impact on ovarian cancer, there is no cure for advanced stages of ovarian cancer. Specifically, there is an urgent need to identify therapies that provide longer-lasting relief for patients with normal BRCA genes, and for patients with platinum-resistant cancers.

Over the past few years, clinical trials have revealed promising treatment options, including Immunotherapy, vaccines and cell cycle checkpoint inhibitors.

Immunotherapy:

• Active immunotherapy aims to stimulate an anti-tumor response from the patient’s own immune system, and thus induce immunological memory
• Passive immunotherapy includes the administration of immune components that promote the anti-tumor response
• Immunomodulation includes all approaches that enhance immune responsiveness¹⁶.

Vaccines: Cancer vaccines are immunotherapeutics proposed to “educate” and activate the immune system to generate specific effector T-cells capable of detecting and killing tumor cells. The ideal tumor-associated antigens should not be present in normal tissue, while being overexpressed in tumor cells. These tumor-associated antigens should also be easily recognized by the immune cells and therefore trigger the immune response also at low doses (strong immunogenicity)¹⁶. Some of the common cancer vaccines are developed and tested using different methods such as epigenetics and genetics, and can be administered alone, or in combination with cytokines or other accelerating factors³.

Cell cycle checkpoints: A few cell cycle checkpoint inhibitors are already in clinical testing or under development. Following DNA damage, cells initiate multiple responses to protect the genome and ensure survival¹⁷. One of these mechanisms includes the activation of specific cell cycle checkpoints that block the cell cycle, allowing the cell to repair its DNA. HGSOC, for example, is characterized by a mutant or null p53 which leads to the dysfunction of the p53-dependent G1/S phase checkpoint. Consequently, HGSOC relies on G2 checkpoint arrest to facilitate DNA damage repair¹⁸ . This emphasizes the importance of therapeutics that specifically target the G2/M phase checkpoint.

With multiple treatment options available for patients, prognostic biomarkers that correlate to disease progression are critical. As previously mentioned, the main factor responsible for OC’s high mortality is the high rate of advanced stage disease at initial diagnosis⁵. Specifically, as the disease progresses, treating and managing patients becomes more challenging³. It is therefore critical to identify predictive prognostic biomarkers for patient response to treatment in order to establish effective treatment strategies and increase overall survival. Currently, commonly used biomarkers to monitor recurrence and predict prognosis are Cancer Antigen 125 (CA125) and Human Epididymis Protein 4 (HE4)³.

CA125: Expression of CA125 is elevated in 85% of serous, 65% of endometrioid, 40% of clear-cell, 36% of undifferentiated and only 12% of mucinous ovarian cancers¹⁹. For the past three decades, CA125 has become the most widely used tumor marker in ovarian cancer, although its limitations in sensitivity and specificity negatively affect its performance¹⁹. Furthermore, CA125 is known to be elevated in only 50% of early stage OC patients, and in about 80% of advance stage OC patients²⁰

Given the aforementioned limitations of CA125, many efforts have been made to identify additional or complementary biomarkers for OC.

HE4 (human epididymis protein 4) is present in the epithelium of fallopian tubes, endometrium, endocervical glands, but not in ovarian surface epithelium. Elevated expression levels of HE4 have been observed in 93-100% of serous, 80-100% of endometrioid and 50-83% of clear-cell carcinomas of the ovary, while HE4 is absent in mucinous ovarian cancer ^21,22

It has been demonstrated that HE4 mRNA levels are higher in ovarian cancer tissue compared to benign^22,23. Specifically, Moore and colleagues showed that HE4 is indeed a useful single marker in differentiating benign versus ovarian cancer patients²⁴. Furthermore, HE4 is also highly expressed in epithelial ovarian cancer, which accounts for 85-90% of ovarian cancer among the various pathological types²⁵. In recent years, HE4 has also become an interesting marker to monitor disease progression, and predict prognosis²³. Studies have confirmed the effectiveness of HE4 in the preoperative assessment of patients with ovarian cancer²⁶. Yang and colleagues provided evidence that preoperative serous HE4 may be regarded as an index to estimate the likelihood of successful cytoreductive surgery in women with ovarian cancer. Specifically, by using a demarcation criterion of 600 pmol/l (values greater than 600 pmol/l indicate lower possibility of optimal debulking by cytoreductive surgery), HE4 predicted incomplete cytoreductive surgery with a sensitivity of 77% and a specificity of 32% ²³. These results suggested that higher levels of HE4 in OC patients are linked to worse prognosis.

In the past few decades, CE CT has played an important role in both pre-operative staging and follow-up, due to the cost-effectiveness and the availability of this technique²⁷. In preoperative staging, CE CT is capable of determining the presence of a malignant adnexal mass, as well as the extent of the disease, two very crucial aspects for treatment planning. CE CT is also an effective tool to assess treatment response, and disease recurrence²⁸.

In 2013, Manganaro and colleagues were able to correlate CE CT results with HE4 levels. Specifically, they found that HE4 serum levels, combined with CE CT imaging, could improve the monitoring management of women affected by EOC²⁸. This represents an important finding, since CE CT imaging is known to cause considerable side effects due to ionizing radiation and contrast medium injections²⁸. Therefore, the use of biomarkers such as HE4 for follow-up as an alternative to imaging, might significantly benefit the patient.

HE4 might also be a promising additional tool for the evaluation of advanced high-grade serous carcinoma (HGSC) patient’s response to neoadjuvant chemotherapy (NACT)²⁹. HGSC is the most common and deadliest type of ovarian cancer, with more than two-thirds of HGSCs diagnosed at an advanced stage³⁰.

The primary standard treatment for EOC, as previously mentioned, is cytoreductive surgery followed by chemotherapy. An alternative approach includes the use of NACT, followed by interval debulking surgery (IDS). In 2014, Vallius and colleagues showed that an 80% variation of HE4 levels during NACT was associated with poor prognosis, thus suggesting that HE4 change during NACT correlates with overall survival (OS). Pre-IDS HE4 and CA125 values predicted IDS outcome in primarily inoperable patients²⁹. Taken together, these results suggest that HE4 may be a helpful tool when deciding whether to proceed to IDS or to second-line chemotherapy²⁹.

A recent longitudinal study by Salminen and colleagues also confirmed the possibility of using HE4 (as a complement to CA125) in HGSC treatment monitoring and in the prognostic stratification of patients31 . Specifically, in this prospective clinical study HE4 was indeed an effective tool in the evaluation of cytoreductibility and the prognostic stratification of patients at disease progression. The investigators found that the measurement of HE4 levels could aid clinicians in detecting platinum resistant disease, while offering support for the determination of individual targeted therapies. HE4 performed equally well to, if not better than, CA125 in all of the time-points evaluated: at baseline, nadir and in 1st relapse³¹.

Furthermore, a prospective study by Angioli looked into changes in serum CA125 and HE4 levels at the beginning, and during, first-line chemotherapy, in order to find correlations of these biomarkers with platinum-based chemotherapy response. They found that, contrary to CA125, HE4 profile was significantly associated with platinum-based chemotherapy response. Additionally, the time required for HE4 normalization during initial chemotherapy, was a valid indicator for the identification of patients who are not responding to treatment, after the third cycle of chemotherapy. Specifically, HE4 levels were >70 pmol/L after primary surgery and three cycles of chemotherapy³².

Similarly, an interesting study by Potenza and colleagues looked into the correlation of serum CA125 and HE4 levels prior to each chemotherapy cycle together with their response curve to the clinical response to treatment, platinum-sensitivity, and progression-free survival (PFS). They found that HE4 decreases more rapidly during chemotherapy than CA125. CA125 had a delay of 21 days. Furthermore, HE4 was redetected faster than CA125 in patients who experienced a poor chemotherapy response²⁶.

Finally, other studies have also suggested that the regular measurement of HE4 levels during the firstline treatment of ovarian cancer may be of prognostic significance. Specifically, the normalization of HE4 levels at end of treatment, and the 50% reduction of HE4 concentrations before interval cytoreductive surgery, are both important and strong predictive factors of time to progression and overall survival time³³ .

Taken together, these results suggest that HE4 is a promising and effective tool for monitoring the response to chemotherapy in ovarian cancer patients.

Despite the continued efforts and steady improvements in ovarian cancer treatment, this disease still remains the deadliest malignancy in women. Specifically, the poor clinical outcome is due to the deficiency of effective tools for detecting the disease at an early stage, chemotherapy resistance and increased heterogeneity of the disease³. Thus, identifying biomarkers that help monitor response to treatment and predict prognosis, can indeed represent an effective tool for the improvement of EOC survival.

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