Oral and injectable Marsdenia tenacissima extract (MTE) as adjuvant therapy to chemotherapy for gastric cancer: a systematic review

Background Marsdenia tenacissima extract (MTE) is a phytochemical widely used as complementary therapy in cancer care. This systematic review was conducted to investigate the anticancer and detoxification effects of MTE, as an adjuvant therapy to chemotherapy, for treating gastric cancer. Methods Ten databases were searched to identify randomized controlled trials (RCTs) comparing oral or injectable MTE plus chemotherapy versus chemotherapy alone for treating gastric cancer up to May 1, 2019. In meta-analyses, proportional odds ratios (PORs) with 95% confidence intervals (CIs) were pooled for the ordinal outcomes using the generalized linear model, and risk ratios (RRs) with 95% CIs were pooled for dichotomous outcomes using the Mantel-Haenszel method. Results Seventeen RCTs with 1329 individuals were included, with a moderate to high risk of selection and performance bias. Compared to chemotherapy alone, MTE adjuvant therapy significantly improved the response to anticancer treatment (POR 2.01, 95% CI 1.60–2.53) and patients’ performance status (POR 3.15, 95% CI 2.22–4.48) and reduce the incidences of chemotherapy-induced leukopenia (RR 0.66, 95% CI 0.56–0.78), thrombocytopenia (RR 0.64, 95% CI 0.48–0.86), anemia (RR 0.89, 95% CI 0.72–1.10), nausea/vomiting (RR 0.79, 95% CI 0.69–0.91), hepatic injury (RR 0.77, 95% CI 0.61–0.96), and peripheral neurotoxicity (RR 0.77, 95% CI 0.59–1.01). However, MTE did not significantly alleviate anemia, diarrhea, constipation, kidney injury, and oral mucosal lesions after chemotherapy. Incidence of nausea/vomiting was lower in patients receiving oral MTE than those receiving injectable MTE (RR 0.47 vs. 0.82, interaction P = 0.04). Heterogeneity was generally low among these outcomes. Three out of five RCTs that reported survival data supported the effects of MTE for prolonging progression-free and/or overall survival. No studies reported safety outcomes of MTE. Conclusions The current evidence with limitations of risk of selection and performance bias suggests that MTE, as an adjuvant therapy to chemotherapy, is effective for inhibiting cancer growth and reducing incidences of multiple chemotherapy side effects. Oral MTE may be a better choice. Uncertainty remains regarding the effects of MTE on survival endpoints and the subgroup differences between acute and chronic use of MTE and between different chemotherapy regimens.


Background
Gastric cancer is a malignant disease that seriously threatens human health and affects the life expectancy, the global annual incidence of which was approximate 12.1 per 100,000 population [1]. China carries a high burden of gastric cancer which occurred in up to 31.38 per 100,000 people in 2013, representing the third leading cause of cancer-related death (mortality 14.54/100,000) [2]. As a basic treatment, chemotherapy can be used for both patients with early and advanced gastric cancer [3]. However, based on chemotherapy, recurrences still occur in 1.9% of the patients with early gastric cancer after radical gastrectomy [4], and the patients with advanced or recurrent gastric cancer only have a median overall survival (OS) of 20.4 weeks [5]. Moreover, patients who received chemotherapy probably experience toxic side effects, such as gastrointestinal reactions, myelosuppression, and hepatic injury, which substantially reduce patients' quality of life and even cause life-threaten complications (e.g. acute infections) [6].
Clinicians, therefore, hope to find complementary and alternative approaches for improving the anticancer efficacy and reducing the chemotherapy side effects in the treatment of gastric cancer. Currently, phytochemicals derived from herbal medicine have been developed and used for a complementary and alternative therapy in cancer care worldwide [7]. Multiple phytochemicals have been proven to be effective for anticancer, such as alkaloid, benzopyran, and coumarin [8].
Marsdenia tenacissima (family: Apocynaceae) is a representative anticancer herb in traditional Chinese medicine that was initially identified by Lan Mao and documented in Medicinal Plants in Southern Yunnan (Dian Nan Ben Cao) in Ming Dynasty (600 years ago) [9]. Based on the empirical evidence in several centuries of traditional medicine practice, the stems of M. tenacissima is expected to be promising for treating cancer (e.g., lung, esophageal, and gastric cancer) and alleviating chemotherapy-induced adverse effects [10]. In China, the M. tenacissima extract (MTE) from the stems has been made into oral or injectable preparations, which is named Xiao-ai-ping [11]. Many animal studies of gastric cancer have revealed that MTE can suppress the growth of cancer cells by inhibiting angiogenesis, eliminating free radicals, and inducting cancer cell apoptosis [12].
Many randomized controlled trials (RCTs) recruiting human subjects evaluated the efficacy of MTE on gastric cancer. The results of these RCTs, however, were inconsistent, which may be attributed to their small sample size and inter-study heterogeneity (e.g., different preparations of MTE [13 14] and different regimens of chemotherapy [15 16]). So far, the effects of MTE as an adjuvant therapy to chemotherapy for treating gastric cancer have not been established. Hence, we conducted a systematic review to inform clinical practice of MTE for gastric cancer by critically assessing and qualitatively synthesizing the current RCT evidence.

Methods
We reported this systematic review in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement (Additional file 1) [13].

Literature search
The relevant literature evidence was searched in ten electronic databases, including PubMed, EMBASE, CEN-TRAL, ScienceDirect, Scopus, Sinomed, China National Knowledge Internet, Wanfangdata, CQVIP, and Clinicaltrials.gov, from their inception to May 1, 2019. The detailed search strategies in each database are presented in Additional file 2. The reference lists of relevant reviews were also checked for acquiring complementary eligibility.

Eligible criteria
An eligible study should be an RCT that compared MTE plus chemotherapy versus chemotherapy alone for treating patients with gastric cancer and reported data on at least one of the outcomes of interest. The patients should be diagnosed as gastric cancer by histopathological examination. No restrictions were imposed on publication language, stage of gastric cancer, regimen of treatment, and length of follow up. We excluded studies that used any other traditional Chinese medicine in either group, assessed outcomes using unclear standards, or had insufficient data for data analysis.

Outcomes
We assessed the following outcomes of interest: (1) Response to treatment assessed by the Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 [14]. The response to treatment was graded as four ranks: 1) complete response (CR): all target lesions disappeared; 2) partial response (PR): the total dimension of the target lesions decreased by 30% or more compared with baseline; 3) progressive disease (PD): new lesions developed or the total dimension of the target lesions increased by 20% or more; 4) stable disease (SD): changes in the target lesions did not meet any of the above criteria.

Study selection and data extraction
Two reviewers, in pairs and independently, read titles and abstracts to identify preliminarily eligible studies and then read full texts to determine the final inclusions.
The following data of the included studies was extracted using a standardized form with a pilot test: title, author, publication year, gender, mean age, stage of gastric cancer, preparation of MTE, regimen of chemotherapy, course of treatment, length of follow-up, and outcome data. Any disagreements were settled by inter-reviewer discussion or consultation with a third reviewer.

Risk of bias assessment
We evaluated the following domains of bias for each RCT referring to the Cochrane risk of bias tool [17]: 1) selection bias (inappropriate random sequence generation and allocation concealment); 2) performance bias (unblinded patients and clinicians); 3) detection bias (unblinded outcome assessors); 4) attrition bias (incomplete outcome data); 5) reporting bias (selective reporting of outcomes); and 6) other bias (e.g., imbalanced baseline characteristics). Each domain was judged to be at low, high, or unclear risk. Two reviewers assessed the risk of bias independently and in duplicate and crosschecked the results. Any discrepancies were addressed by inter-reviewer discussion or consultation with a third reviewer.

Data analysis
Frequency and incidence were used as the descriptive statistics for all outcomes. The dichotomous outcomes were measured by risk ratios (RRs) with 95% confidence intervals (CIs) and pooled by the Mantel-Haenszel method. The ordinal outcomes (i.e., response to treatment and performance status) were measured by proportional odds ratios (PORs) with 95% CIs. We first calculated the natural logarithm of the POR (logPOR) and its standard error (selogPOR) for each RCT using the generalized linear model and then pooled the individual results by the generic inverse variance method [18]. Heterogeneity across studies was quantitatively assessed by the I 2 statistics, and an I 2 > 50% indicated a significant heterogeneity. Given that there were always clinical and methodological varieties among the included RCTs, we performed all meta-analyses under a random effects model. To explore the cause of heterogeneity, we performed a set of subgroup analyses stratified by the different preparations of MTE (oral versus injectable). An interaction P < 0.05 indicated a significant betweensubgroup difference. Funnel plots and Egger's tests were used to examine the presence of publication bias for each outcome. Duval and Tweedie's trim and fill test was used to adjust the results with significant publication bias [19]. SAS v9.4 (SAS Institute Inc., NC, USA) was used to calculate the PORs and perform the tests for publication bias; RevMan v5.3.5 (St. Louis, Missouri, USA) was used to perform the meta-analyses and draw the funnel plots.

Level of evidence assessment
The Grading of Recommendations Assessment, Development and Evaluation (GRADE) instrument was used for assessing the level of evidence for the outcomes with meta-analytic result. Five aspects of limitation of evidence were assessed, including risk of bias, imprecision, inconsistency, indirectness, and publication bias.

Study description
The literature search identified 328 records, and 17 RCTs [13- 16 24-36] involving 1329 patients were finally included after the screening (Fig. 1). Among the included RCTs, the overall proportion of male was 55.9% and the mean age ranged from 51.5 to 68.2 years. Two trials recruited patients with early gastric cancer and the rest recruited patients with advanced gastric cancer. Injectable MTE was used in 13 trials and oral MTE in 4 trials. MTE was administered as an acute treatment (injectable MTE: 40-80 ml/d, 7-21 d/session, 2-4 sessions; oral MTE: 6-7.2 g/d, 30 d/session, 2 sessions) along with the chemotherapy in all trials. The most common chemotherapy regimen was FOLFOX (folinic acid + fluorouracil + oxaliplatin, 5 trials), followed by XELOX (capecitabine + oxaliplatin, 4 trials). Table 1 presents the study characteristics in details.

Risk of bias
As shown in Fig. 2, all RCTs were considered to be at a moderate to high risk of bias. Specifically, five RCTs [27,31,32,34,35] generated the allocation sequence using a random number table, and the others did not report the method of allocation sequence generation. No RCTs reported information on allocation concealment and blinding of patients, clinicians, and outcome assessors. Four RCTs [26,29,32,35] lost a few (1 to 6) patients during the follow-up, while the others had a complete followup. Three RCTs [24,28,32] seemed to have selective reporting since they did not report all planned outcomes or did not provide sufficient outcome data.

Safety
No studies reported safety information regarding MTE.

Subgroup analysis
The results of subgroup analyses stratified by the different preparations of MTE are presented in Table 2. Patients who received oral MTE had a significantly lower incidence of nausea/vomiting compared with those who received injectable MTE (RR 0.47 vs. 0.82, interaction P = 0.04). No significant subgroup difference was found for other outcomes, indicated by an interaction P > 0.05.

Publication bias
Based on the results of the funnel plots and Egger's tests, publication bias was considered to be significant for the response to treatment (Egger's test P = 0.037) but not significant for the incidences of leukopenia (P = 0.225), thrombocytopenia (P = 0.778), nausea/vomiting (P = 0.971), hepatic injury (P = 0.466), and peripheral neurotoxicity (P = 0.121). The adjusted analysis using the trim and fill tests for the response to treatment did not show apparent changes (POR 1.90, 95% CI 1.50-2.42). The tests for publication bias were not available for the other outcomes due to insufficient sample size.

Level of evidence
The level of evidence assessment showed that all outcomes did not suffer serious limitation on inconsistency, indirectness, and publication bias but suffered serious to very serious limitation on risk of bias and/or imprecision. As a result, four (leukopenia, thrombocytopenia,  nausea/vomiting, peripheral neurotoxicity), six (response to treatment, performance status, anemia, diarrhea, hepatic injury, and oral mucosal lesions), and two outcomes (constipation and kidney injury) were judged as moderate, low, and very low level of evidence, respectively (Table 3).

Discussion
This systematic review was conducted to evaluate the effectiveness of MTE against gastric cancer. The results revealed that MTE, as an adjuvant therapy to chemotherapy, improved the response to anticancer treatment and patients' performance status, and meanwhile, reduced the incidences of leukopenia, thrombocytopenia, nausea/vomiting, hepatic injury, and peripheral neurotoxicity induced by chemotherapy. However, MTE did not significantly alleviate anemia, diarrhea, constipation, kidney injury, and oral mucosal lesion after chemotherapy. The effects of MTE on PFS and OS were uncertain. The response to treatment was assessed by the RECIST criteria, which focused on the changes in the dimension of cancer lesions. Hence, the findings on the response to treatment imply that MTE could inhibit the growth of gastric cancer cells. Based on the current in vitro and animal research evidence, phenolic acid, C-21 steroidal glycosides, and polyphenols in MTE may play a critical role in its anticancer mechanism, which could suppress angiogenesis in cancer tissues by blocking the activation of vascular endothelial growth factor  receptors and phosphorylated protein kinases [12,37,38]. MTE also has effects of prolonging mitosis cycle and inducing apoptosis for cancer cells, which may be modulated by multiple factors, such as phosphoinositide 3-kinases, protein kinase B, mammalian target of rapamycin, and extracellular regulated protein kinases [39].
The incidences of multiple chemotherapy side effects were reduced after the MTE treatment, especially for thrombocytopenia (− 36%) and leukopenia (− 34%), with acceptable heterogeneity. Similar effects of MTE for reducing the adverse effects of chemotherapy have been reported in previous research [39,40], but the  underlying mechanism is unclear since there is a lack of relevant pharmacological studies. The subgroup analysis suggested a favorable effect on relieving nausea/vomiting for the oral MTE compared with the injectable MTE.
Given that there was comparable efficacy on the response to treatment and performance status between the oral and injectable preparations, oral MTE appears to be a better choice. However, this finding needs further evidence because the subgroup analysis included only two RCTs of oral MTE. In cancer research, PFS and OS are both important long-term endpoints. Although significant anticancer effects were found for the surrogate outcomes (i.e., the response to anticancer treatment and performance status) in our review, whether MTE will ultimately prolong PFS and OS still lacks evidence. The results of PFS were inconsistent across the included RCTs; and for OS, the sample size (74 in each group) is too small to yield a definite conclusion. Moreover, even if that the between-group difference was statistically significant, the absolute estimates (mean difference of survival time) would be only approximately one month for median PFS and two months for median OS based on the reported data, the clinical implication of which might be limited.
The safety of herbal extracts has been a target of public criticism [41]. We were unable to assess the safety of MTE because no such information was reported. Currently, there are no reports of acute or subacute toxicity of oral MTE. A rat study reported that oral MTE did not cause any toxicity effects or outcomes at an acute toxicity dose of 5 g/kg body weight for 14 days and subacute doses of 0.25, 0.5, and 1 g/kg body weight for 28 days [42]. Another rat study also showed that an acute oral dose of 2 g/kg body weight MTE did not cause any deaths up to 2 days [43]. However, some observational human studies have reported a number of adverse events in malignant patients caused by MTE, such as rash, shiver, chills, malaise, nausea, abdominal pain, and Marsdenia tenacissima extract for reducing adverse effect of chemotherapy Note: In GRADE instrument, the level of evidence of outcomes is judged as high, moderate, low, and very low if the total score is "-0", "-1", "-2", and "≥ −3", respectively palpitation, all of which were determined to be associated with MTE by the Provincial Food and Drug Administrations and most of which occurred within 60 min after the administration and when MTE was administered as an injection [44,45]. Therefore, the administration of MTE-injection in particular-needs a close observation of patients' adverse reactions during the medication.
A previous systematic review has assessed the effect of MTE on gastric cancer [46]. However, the review was only focused on the injectable MTE and advanced gastric cancer, assessed less chemotherapy-induced side effect outcomes, and suffered some methodological limitations in its data analysis of primary outcomes. For example, its conclusions were predominately drawn from the subgroup analyses stratified by different chemotherapy regimens, but the subgroup analyses did not include all regimens, which was an incorrect way. The results of tests for subgroup difference (i.e., interaction p values) were also not considered in the interpretation of the subgroup effects. These limitations finally misled the subgroup findings --the review found that the anticancer effects of injectable MTE were significant in patients receiving XELOX but not significant in patients receiving FOLFOX and S-1 + oxaliplatin in both main outcomes, while these subgroup differences were actually false-positive that can be explained by chance because the interaction p values were 0.40 and 0.78 (> 0.05), respectively [47]. Furthermore, the review had no appraisal on the quality of evidence for the outcomes.
Compared with the previous review, our systematic review included additional four RCTs, assessed and compared oral and injectable MTE, and reported more chemotherapy-induced side effect outcomes, including thrombocytopenia, anemia, diarrhea, constipation, peripheral neurotoxicity, and oral mucosal lesions. Our systematic review also has several strengths in methodology. First, we defined consistent criteria (i.e., the RECIST criteria and the Karnofsky scale) to assess the response to treatment and performance status, which facilitated lowering the heterogeneity and interpreting the results. Second, the previous review used an inappropriate method to analyze the ordinal variables in which the ordinal variables were converted to be dichotomous by combining adjacent values using a cut-point. This method lost the difference within the combined values and may bias the results [45,48]. Oppositely, we calculated the PORs using the generalized linear model by assuming that the odds ratios were proportional for all dichotomies of the values, which can maximize the information utilization and yield more reasonable results. Third, the overall heterogeneity was low in the metaanalyses, and part of it was explained by the subgroup finding (oral versus injectable MTE) with an interaction p value less than 0.05-the reliability of the relevant results was thus improved. Fourth, we critically appraised the quality of evidence for each outcome using the GRADE instrument, which increased the precision and applicability of the findings for the clinical practice and guideline development.
There are some limitations in this review due to the inherent deficiency of the included RCTs. First, because all RCTs were at a high risk of selection and performance bias, the results may be impacted by inadequate randomization and placebo effects, which substantially weakens the level of evidence of all outcomes. Second, acute or chronic use of MTE and different chemotherapy regimens may be important causes of heterogeneity. However, we did not perform these subgroup analyses due to insufficient data or potentially high probability of type I error induced by a large number of subgroup hypotheses [47]. Third, the result of the response to treatment suffered significant publication bias, which usually lead to an overestimation. Nevertheless, the adjusted analysis using the trim and fill method did not show substantial changes, suggesting that the publication bias should not significantly deviate the estimate.

Conclusion
The current evidence suggests that using MTE as an adjuvant therapy to chemotherapy may improve the response to anticancer treatment and performance status in patients with gastric cancer. MTE may also reduce several chemotherapy adverse effects. Oral MTE may be a better choice. The reliability of these findings, however, is limited by the high risk of selection and performance bias across the included RCTs. Uncertainty remains regarding the effects of MTE on survival endpoints and the subgroup differences between acute and chronic use of MTE and between different chemotherapy regimens. Large-sample, long-term, double-blinded RCTs with reporting of any safety outcome are warranted to provide high-quality evidence on the efficacy and safety of MTE for treating gastric cancer.

Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Ethics approval and consent to participate
This study was based on previously published studies; therefore, ethical approval and patient consent are not relevant.