We found that the initial dose of q3w nab-PTX had a significant effect on taxane-related QoL and cancer fatigue. QoL of patients with metastatic breast cancer treated with 260 mg/m2 of q3w nab-PTX was significantly worse compared to those treated with 220 mg/m2 or 180 mg/m2 of q3w nab-PTX. Primary endpoint analysis showed that intravenous administration of low-dose nab-PTX at 180 mg/m2 q3w may be the optimal therapy with meaningful efficacy and favorable toxicity for patients with metastatic breast cancer [4]. The QoL substudy also showed that low-dose nab-PTX at 180 mg/m2 q3w is optimal in terms of QoL and cancer-related fatigue.
Standard doses of chemotherapeutic agents should be determined based on risk and benefit balance. However, in terms of dose intensity, the standard dose is often determined by acute toxicities such as hematologic toxicity, diarrhea, and vomiting during development of the drug. Regarding the cumulative toxicity of chemotherapy, a strategy of continuing treatment by short-term drug withdrawal or reducing the dose is used based on observation of adverse events. However, some adverse effects of chemotherapy are not easily improved by withdrawing or reducing the dose of chemotherapy, and CIPN is an example. In a discussion at the start of the study, treatment with nab-PTX using drug reduction and withdrawal was suggested to be optimal. However, we obtained a result that overturned this opinion, since we found that the initial dose of nab-PTX has a significant effect on subsequent patient QoL.
CIPN is a major clinical challenge due to lack of effective treatment and impact on QoL [9]. There are no drugs for preventing or ameliorating CIPN in a long-term course, and chemotherapy schedule modification is often required to limit its severity; however, this also may limit the efficacy of the cancer treatment. Moreover, symptomatic therapy is often ineffective in reducing CIPN symptoms [10]. Taxanes are representative chemotherapeutic agents that induce CIPN. Taxanes inhibit depolymerization of microtubules [11, 12], and CIPN may be caused by the resulting abnormal aggregation of microtubules in neuronal cells. Polyoxyethylated castor oil, which is used as a vehicle for taxanes, may also increase the risk for prolonged peripheral neuropathy [13]. Animal studies have shown that the time to recovery from CIPN depends on the drug, and that the morphological effect on axons in PTX-treated animals is long lasting [14].
Nab-PTX is an albumin-bound, nanoparticle formulation of PTX that was developed to reduce the incidence of CIPN compared to that with conventional taxanes [15, 16]. However, in the CA012-0 trial, a phase III study comparing 3-week cycles of nab-PTX (260 mg/m2) and so-PTX (175 mg/m2) in patients with metastatic breast cancer, CIPN occurred more frequently with nab-PTX, despite nab-PTX showing greater efficacy in terms of RR [1]. In contrast, superiority of nab-PTX over so-PTX in PFS was not found in patients with metastatic breast cancer in the CALGB40502 phase III trial, and hematologic toxicity of grade ≥ 3 and CIPN occurred more frequently with nab-PTX [17]. Dose reduction was more common and occurred earlier with nab-PTX, and discontinuation rates were also higher. Therefore, it was concluded that the dose of nab-PTX used in the study (150 mg/m2/week) was not appropriate and resulted in significant toxicity. These studies indicate that the initial dose of nab-PTX can cause adverse events that may have a long recovery time and a significant effect on patient outcomes, consistent with the findings obtained in this study.
Cancer-related fatigue causes disruption of all aspects of QoL and may be a risk factor for reduced survival [18]. Fatigue may be elevated before treatment onset and typically increases during cancer treatment, including that with radiation, chemotherapy, hormonal, and/or biological therapies. The rate of cancer-related fatigue ranges from 4–91%, depending on the population studied and the methods of assessment [19]. In a RCT comparing taxanes and S-1 in patients with metastatic breast cancer in Japan, the frequency of fatigue in the taxane group was approximately 50% in all grades [20]. However, these data are based on physician’s assessments, which may be underreported, and the actual rate of fatigue in taxane-based chemotherapy for metastatic breast cancer is likely to be higher [21]. Fatigue in cancer patients is multifactorial and may be influenced by demographic, medical, psychosocial, behavioral, and biological factors [18]. Nevertheless, the initial dose of nab-PTX had a significant effect on subjective fatigue in this study, especially for physical fatigue. At the current standard dose of 260 mg/m2, physical fatigue worsened over the course of treatment, but no worsening tendency was observed at 220 or 180 mg/m2.
Both CIPN and fatigue are symptoms that are difficult for physicians to evaluate accurately, and in clinical trials, the discrepancy between physicians’ assessments and patients reported outcomes (PROs) can be a problem [21]. In a RCT of adjuvant chemotherapy with taxanes for breast cancer, a substudy of the agreement between physician and patient assessments for CIPN gave kappa coefficients for sensory and motor symptoms of 0.16 and 0.22, which are extremely low [22]. In a comparative study of clinician and patient assessment of symptoms in lung cancer patients, 41% of clinicians underreported symptoms of fatigue compared to patient reports [23]. Therefore, as the results of this study indicate, there is a limit to the therapeutic strategy of monitoring the cumulative toxicity associated with chemotherapy by physician assessment and reducing the dose or withdrawing the drug when this toxicity becomes significant. If a treatment strategy is to truly maintain and improve QoL, it is essential to monitor PROs in daily clinical practice [24]. It is also important to verify the dose that achieves the optimal risk and benefit balance after marketing of a new chemotherapeutic agent, particularly for drugs with cumulative toxicity.
A limitation of this study is that it is based on a secondary endpoint of a small randomized phase 2 trial, with no adjustment for multiple tests. Also, a relatively short-term evaluation up to the sixth cycle was used, and long-term effects were not examined. Furthermore, a weekly schedule of nab-PTX is more commonly used to treat patients with breast cancer, and the significance of our findings for the reduced dose of q3w nab-PTX may be limited. Nab-PTX at 100 mg/m2 can be administered weekly to reduce the incidence and degree of myalgia. Nonetheless, it was not until recently that the weekly nab-PTX regimen was adopted in Japan for treatment of patients with metastatic breast cancer due to previous failures to demonstrate its superiority to q3w docetaxel in terms of PFS [25]. There should be room for less frequent regimens, especially in patients with limited access to clinics or in those who might benefit from minimizing the risk of coronavirus infection during the COVID-19 pandemic.
We conclude that the initial dose of nab-PTX affects subsequent taxane-related QoL, as well as cancer-related fatigue. Low dose nab-paclitaxel at 180 mg/m2 q3w may be an optimal dose in terms of PFS, taxane-related QoL and cancer-related fatigue. Further evaluation is warranted to confirm these findings in a larger trial.