A variety of clinical studies and experiments have investigated the potential therapeutic effects of VAE and its compounds in breast and gynecological cancer, and predominantly reported positive effects. Nevertheless they have to be interpreted with caution and within their context.
The strongest and most consistent results from VAE in clinical studies concern QoL and improved tolerability of conventional treatment. QoL questionnaires included mostly well established and validated QoL instruments and one on psychosomatic self-regulation. The latter is a 16 item QoL instrument that measures competence and autonomy, in terms of the ability to actively adapt to stressful life situations and to restore well-being.  This tool has so far been exclusively used in studies focusing on complementary cancer treatments. Improvement was seen especially in relation to self-regulation, fatigue, sleep, nausea/vomiting, appetite, diarrhoea, energy, ability to work, enjoyment of life, depression, anxiety, pain, and general physical, emotional, and functional well-being (for more details see Kienle GS, Kiene H: Influence of mistletoe treatment on quality of life in cancer patients. A systematic review of controlled clinical studies. Submitted). Regarding the side effects of conventional oncology treatments, reduced hematopoetic damage (i.e. leukopenia) and immuno-suppression was reported by some, but not by all studies. Similar, less chemotherapy-related events were observed in some but not in all studies. Validity of this evidence is quite good. 15 RCTs are available, four of them double-blinded (three of them showing a positive result) and one with an active control treatment. 5 RCTs reported following ICH-GCP guidelines and three of them comprised more than 200 patients each. Questions remain regarding observation or reporting bias, which is of major importance in relation to subjectively assessed outcomes such as QoL and subjective symptoms. Treatment should therefore be blinded; but blinded subcutaneous VAE application can easily be correctly identified by doctors and patients [55, 137], due to its local reactions and mild flu-like symptoms. In the four blinded trials reviewed here, a considerable degree of unblinding was detected by asking patients and physicians in one study ; and can be presumed in two other of these trials where substantially more VAE-treated patients reported local reactions than control patients [54, 57]. Other RCTs did not blind treatment application, as blinding is unreliable. Therefore questions will remain in "blinded" as well as in open trials even though in general cancer or non-cancer trials could not detect relevant improvements of QoL or disease symptoms due to suggestive administration of inert substances [138–140]. Nevertheless, the frequency, magnitude, duration and conditions of QoL or symptomatic improvement in the course of VAE treatment should be clarified in more detail. Especially relevant might be the further elucidation of possible effects on cancer-related fatigue (see also ), which is one of the most disabling conditions in cancer patients, with only few therapeutic options for influencing it effectively [142–144]. Regarding simple pre-post assessments of QoL in single-arm studies, it is probably unnecessary to state that they are generally not appropriate for judging influences on QoL, since it is affected by many factors.
Concerning survival (Table 3), some of the RCTs show a statistically significant benefit while others show a statistical trend or no difference. Most of the non-RCTs (which included larger patient numbers) show a major impact. The validity of the studies is limited because of their small sample size (median only 52 participants per RCT), and because 8 of the 9 RCTs were imbedded in the same (large) epidemiological cohort study. This study was started in the 1970s, before modern standards of data quality control (ICH-GCP, GEP) were established, and it therefore does not fulfil modern standards in this respect. The 9th RCT had enrolled more patients but was conducted even earlier, and suffers from a major attrition rate due to protocol violation ; the subsequent analysis followed the "as treated" instead of the "intention-to-treat" principle . Hence bias cannot be excluded. None of the survival studies was blinded, but survival is generally not easily affected by observer bias or suggestive effects [138–140]. Seen altogether, although results were consistent, questions regarding survival remain and validity of evidence is moderate at best. An independent, GCP-conform trial with sufficient power would be desirable to further evaluate potential survival benefit.
Regarding tumour behaviour, evidence from RCTs is scanty; most benefits were shown in non-randomized studies. In single-arm studies of patients with no concomitant conventional cancer treatment, high-dose or local application of whole VAE led to substantial remission of tumour or malignant effusion. This was also observed in animal studies: local application resulted in tumour-growth inhibition and increased survival. However, this application and dosage is not standard and cannot be recommended widely due to potential risks of high dose or local application. With ordinary VAE application, schedule and dosage, spectacular tumour remissions tend to be the exception [20, 36]. No tumour remission was observed after application of rMLs. Remission in CIN cannot be distinguished from spontaneous remission rates, which are frequent in this indication.
Apart from the discussed issues, the following validity aspects have to be considered: An attrition rate above 10% was present in 10 RCTs. In 5 of these RCTs [49–51, 53], patients were excluded before baseline assessment. Here the patients were provisionally enrolled into the matching and pairwise randomization procedure; subsequently they were asked for informed consent, and were excluded from the study if they declined, together with their matched twin. Even though the risk of bias with this procedure is small, as the complete randomization unit (patient pair) is excluded, the preferred conservative quality assessment in this review assessed these studies as not having excluded a drop-out bias. Of the remaining 5 trials, one had protocol violations in about 20% of patients as discussed above , and one trial used an aggressive chemotherapy that inevitably had to be halted in several patients . Three trials did not report details.
To reduce publication bias we also included unpublished studies and conducted a thorough literature search with extensive expert consultations. One unpublished RCT (Lektinol in breast cancer by Schwiersch et al.) could not be included as it was not released by the manufacturer. Beyond this, we cannot rule out the existence of unpublished and unknown RCTs, but we presume that no well-conducted, large-size and valid trials escaped our attention. – Regarding preclinical studies achieving completeness is nearly impossible. These experiments are usually explorative, for instance when plant extracts are chemically analysed for active compounds or for cytotoxic effects; in general only relevant results are published, but not results of non-relevant or non-working models or unstable chemicals. (Even in the reviewed experiments, often not all but only the noteworthy results were presented in detail.)
Regarding funding, 27 of 28 controlled studies published since 2000 reported their funding source: 11 studies received funding from the pharmaceutical industry alone, 16 studies (all by Grossarth et al.) had both industry and public funding. There was no difference of results depending on funding source.
Regarding non-RCTs, bias by self-selecting the treatment is usually present in raw data. In particular, patients who choose complementary treatments differ substantially from patients not choosing them [70, 146]. It is therefore indispensable to conduct careful adjustment of baseline imbalances or matching [147–149]. This has been done to a varying degree in most studies except in one without any adjustment , and in another which only adjusted for the main outcome parameter but not for the other reported results . Without any adjustment, no conclusions can be drawn regarding the applied treatment. When conducted and analysed carefully, non-RCTs can provide valuable information regarding external validity and effectiveness, as they can investigate treatment effectiveness under routine conditions without distortion by the artificial and selective conditions of an RCT's experimental situation .
In preclinical studies, VAE show substantial cytotoxic effects in cells originating from breast and gynaecological cancer, and display tumour-growth inhibition in animal studies. Cytotoxicity, especially of the MLs (which bind on human breast cancer cells ), may be the cause of tumour reduction after local, intratumoural application of VAE. If systemically applied, the cytotoxicity of the MLs is of less relevance, as it is inhibited by serum glycoproteins  and by anti-ML antibodies  which are produced after a few weeks of VAE application. Therapeutic effects of the MLs were inconsistent and not very impressive in the reviewed experiments. However, in other tumour types, MLs have also shown substantial growth-inhibiting effects (e.g. [154–157]). Interestingly, in two experiments, the application of VAE-activated macrophages in mice not directly treated with VAE also showed tumour-growth inhibiting effects, while the application of non-activated macrophages had no effects . Similarly in melanoma, the application of VAE-activated splenocytes inhibited metastasis [158, 159].
In general, the predictive reliability of the preclinical studies for clinical application is fairly limited in most instances. Clinical cancer disease is insufficiently mimicked by animal models, with major differences regarding age, general condition, co-morbidity, invasiveness, metastases, antigenicity, immune system etc. The results of preclinical screening, especially for treatment of solid tumours, have therefore been largely disappointing. The models currently regarded as best for cytotoxic substances use patient-derived tumours that grow subcutaneously or orthotopically in nude mice, as in several cases reviewed here. Immuno-active substances may however still be insufficiently assessed in immune-deficient animals, as the main components of the immune system are missing (nude mice, for instance, cannot generate mature T-lymphocytes). Nevertheless, these preclinical experiments can provide important additional information for detecting the possible anti-cancer effects of medicinal plants, their active compounds, their mode of action and potential risks [20, 160–162].
Mistletoe therapy was well tolerated in the reviewed studies. Mild flu-like symptoms and local reactions at the injections sites are frequent, dose-dependent and self-limited. Allergic reactions can occur, and a few case reports of anaphylactic reactions exist [163–166]. A phase I study, conducted at the NCCAM/NCI, investigated safety, toxicity and drug interactions between VAE and gemcitabine  and reported good tolerability, with neither dose-limiting toxicity of the VAE nor any effects on the plasma concentration of gemcitabine . Combination of VAE with chemotherapy or radiotherapy did not negatively influence remission rate in clinical and in animal studies [56, 63, 118]. A higher prevalence of depression in VAE-treated patients in one study was observed in raw data of a self-selected population, without adjustment of baseline imbalances. This difference can be ascribed to variations in the patient population; for instance, they differed markedly in the prevalence of hormone treatment. No toxicity was observed in animal experiments.