6. Discussion
6.1 Patient characteristics, overall response, and toxicity profile
6.3.8 Multivariate analysis
Among all the groupings with P <0.05, only grouping 11110 (ie, neutrophil < 5000
/µl at least once) retained significance in multivariate analysis. This could suggest that
neutropenia was a stronger prognostic factor than GI toxicity. But due to the low case
number of this study, the result of Cox regression was less plausible.
6.4 Short-term analysis
Clinical observations of relationship between toxicity and efficacy mostly
involved short-term efficacy, which means that with occurrence of toxicity, stronger
tumor-killing effect would be anticipated. The purpose of short-term analysis was to
verify the clinical impressions, and may offer some guidance for clinical decisions, as
failure to achieve remission after a treatment might be the consequence of absence of
toxicity. In this study, short-term analysis demonstrated that occurrence of toxicity
actually increased the like hood of effective treatment over ineffective treatment.
However, the intrinsic limitation of short-term analysis lies in that if a patient was in
remission or having stable disease, it could be chemotherapy suppressing the tumor or
the tumor simply not yet relapsing into gross disease. Hence, many treatments were
assigned as uncertain treatments, and despite the above result, it was uncertain
treatments that accounted for the largest proportion of all treatments in nearly every
toxicity status, which could diminish the usefulness of the findings.
6.5 Toxicity-adjusted dosing
The primary goal of investigating correlation between toxicity and efficacy was to
develop a more appropriate dosing system, since absence of toxicity implying
underdosing and could cause less favorable treatment outcome. Many studies
demonstrating a positive relationship between toxicity and efficacy mentioned that a
prospective dose escalation trial could further strengthen the relationship (Mayers et al.,
2001; Stintzing et al., 2011; Vaughan et al., 2007), or even advocated clinical
applications of increasing traditional doses given lacking of toxicity (Cameron et al.,
2003; Carpenter et al., 1982; Di Maio et al., 2005; Saarto et al., 1997; Sorenmo et al.,
2010). Gao et al (Gao et al., 2008) named this dosing strategy as toxicity-adjusted
dosing (TAD), and regarded it as convenient and practical and can be supplemental to
traditional body surface area-based dosing to approach maximum tolerated dose.
Innovative individual dosing methods in human oncology, such as therapeutic drug
monitoring (TDM) and glomerular filtration rate-based dosing, are far from available in
veterinary practice. Toxicity-adjusted dosing could be the most feasible method to
improve the current dosing system for veterinary oncology patients. The results of this
study not only supported but also offered guidance to this strategy: Achieving
neutrophil nadir to lower than 5000 /µl could be set as a goal of treatment, whereas dose
modifications according to GI toxicity could be risky, as high-grade GI toxicity did not
produce better outcome, and high-grade diarrhea even produced poorer outcome.
Nevertheless, increasing dose due to absence of neutropenia might also cause more GI
toxicity, adding uncertainty to this approach, and it was hard to decide if adjusting dose
based on GI toxicity was truly a suboptimal approach since low-grade GI toxicity
actually increased survival. Due to the doubts discussed above, implementing
prospective, controlled clinical trials would assist to develop a more sophisticated
toxicity-adjusted dosing algorithm for canine lymphoma. In addition, similar studies
focusing on other malignancies could also prompt generalizations of toxicity-adjusted
dosing strategy to other tumors.
6.6 Limitations
The retrospective nature of this study caused the primary limitations. Above all,
inadequate blood sampling frequency was a major issue when assessing neutropenia.
Ideally, blood sampling should be done on the 7th day after vincristine,
cyclophosphamide, and chlorambucil and at the 14th day after doxorubicin. However,
this was not always accomplished even in the first two cycles of the protocol and rarely
accomplished beyond the first two cycles. In this study, blood sampling 7 to 14 days
after vincristine, cyclophosphamide, and chlorambucil, and 14 to 28 days after
doxorubicin was regarded as acceptable for assessing bone marrow toxicity. The true
incidence of neutropenia could be underestimated under the loose criterion. In addition,
because it is a common practice that vincristine injection followed by oral
cyclophosphamide at home, skipping one hospital visit, the true incidence of
vincristine-induced neutropenia could be underestimated more than other drugs.
Detecting GI toxicities from medical records was somehow a minor issue compared
to inadequate blood sampling frequency, as with complete history taking, all GI signs
after last visit would be documented, which accounted for most of the scenarios.
under those circumstances grading of GI toxicities was not accurate enough. Correctly
assigning GI signs as GI toxicities secondary to chemotherapy on the basis of medical
records was also another challenge in the study, since the true causes of GI signs are
sometimes not easy to obtain even in clinical practice. This is an inevitable obstacle for
all studies evaluating chemotherapy toxicity.
Lastly, frequent protocol variations among patients and low case number are also
limitations of the study. Uneven treatment course, an extra variable of comparisons
between patients, decreased the credibility of this study. Although a case number of 69
dogs was moderate in veterinary oncology studies, more case included would decrease
the sampling bias and improve reliability of the study.
7. Conclusion
In long-term analysis, a positive correlation between toxicity and efficacy was
found for some forms of toxicity: Neutrophil nadir lower than 5000 /µl improved OST;
Vomiting or combined GI signs of any grade improved TTP and OST. High-grade
toxicity was not proved to be of more benefits than low-grade toxicity: Patients with
neutrophil nadir lower than 1500 /µl had similar survival to patients with Neutrophil
nadir of 1500 ~ 5000 /µl, although improved OST in high-grade toxicity group could be
subjectively observed in the Kaplan-Meier’s curve; Patients with high-grade vomiting
or combined GI signs had similar outcome to patients with low-grade vomiting or
combined GI signs, although poorer outcome in high-grade toxicity group could be
subjectively observed in the Kaplan-Meier’s curves; Patients with high-grade diarrhea
had poorer outcome than both patients with low-grade diarrhea and no diarrhea. No
drug was found to induce stronger relationship between toxicity and efficacy.
Frequency and timing of toxicity were not determinants for relationship between
toxicity and efficacy. More than 10 years old was associated with less low-grade GI
toxicity and decreased survival, but the causal connections could not be determined. In
short-term analysis, occurrence of neutropenia or GI toxicity after a treatment increased
the like hood of effective treatment over ineffective treatment.
To the author’s knowledge, this is the first report demonstrating the positive
correlation between GI toxicity and outcome, and also the first report analyzing the
relationship between toxicity and short-term efficacy. The results of the study supported
the concept of toxicity-adjusted dosing, but prospective trials are warranted to develop a
sophisticated toxicity-adjusted dosing regimen.
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Table 1 (Revised from Hon et al., 1998)
Human oncology studies establishing relationship between toxicity and efficacy.
Drugs Tumor type Toxicity Effect Reference
Doxorubicin Osteosarcoma Low leukocyte nadir
BEP: Cisplatin + etoposide + bleomycin; CAFt: Cyclophosphamide + doxorubicin + oral ftorafur; CEB: Carboplatin + etoposide + bleomycin; CMF: Cyclophosphamide + methotrexate + fluorouracil; DDFS: Distant disease-free survival; PFS: Progression-free surival; DFS: Disease-free survival; OS: Overall survival.
Table 2
Selected veterinary studies investigating relationship between toxicity and efficacy.
Drugs Tumor
RT: Radiotherapy; LSA: Lymphoma; TCC: Transitional cell carcinoma; Tx: Treatment;
BM: Bone marrow; OS: overall survival; CR: Complete remission; TTP: Time to tumor progression; DFI: Disease-free interval
Table 3
The 6-month, maintenance-free, modified version of the University of Wisconsin (UW)-Madison chemotherapy protocol (UW-25) utilized in this study. (Garrett et al., 2002)
Table 4
Modified neutropenia grading system based on VCOG-CTCAE v1.0.
Table 5
Five-digit coding system for long-term analysis.
Digit Definitions of code
Neutropenia Anorexia; Vomiting;
Diarrhea; GI
Table 6
Neutropenia profile presented as the number and percentage of patients experienced certain type of toxicity of certain grade.
Neutropenia grade
1.0~3 1.0 1.1 2 3
No. % No. % No. % No. % No. %
All cycles 57 83% 47 68% 37 54% 6 9% 4 6%
Cycle 1~2 53 77% 34 49% 31 45% 5 7% 3 4%
Vincristine 47 68% 31 45% 28 41% 4 6% 3 4%
Cyclophosphamide 28 41% 28 41% 9 13% 3 4% 1 1%
Doxorubicin 9 13% 5 7% 2 3% 1 1% 0 0%
No grade 4 neutropenia was documented.
Table 7
GI toxicity profile presented as the number and percentage of patients experienced certain type of toxicity of certain grade.
Anorexia grade
No grade 1 and grade 4 anorexia was documented.
Table 7 cont’d
Table 8
Median TTP and OST and P values for prognostic factor analysis.
Age Sex
N: Numbers of patients in each status; TTP: Time to tumor progression; OST: Overall survival time; T_c1c2: Time to finish the first two cycles of the protocol.
P values <0.05 were printed in bold type.
Table 9
P values for TTP and OST of the 154 groupings in long-term analysis
Grouping 11110 111113 11115 11130 11140 11143 21110 21113 21115 TTP: Time to tumor progression; OST: Overall survival time.
P values <0.05 were printed in bold type.
Table 9 cont’d
TTP: Time to tumor progression; OST: Overall survival time.
P values <0.05 were printed in bold type.
Table 10
Median TTP and OST and P values for 15 groupings regardless of factors of timing, drug, and frequency (ie, second, third, and fifth digit of coding system fixed to 1).
TTP: Time to tumor progression; OST: Overall survival time.
Grouping 11110: No toxicity =No neutropenia; Toxicity =Neutropenia grade 1.0~3, at least once. Grouping 21110: No toxicity =No anorexia; Toxicity =Anorexia grade 1~4, at least once. Grouping 31110: No toxicity =No vomiting; Toxicity =Vomiting grade 1~4, at least once. Grouping 41110: No toxicity =No diarrhea; Toxicity =Diarrhea grade 1~4, at least once. Grouping 51110: No toxicity =No GI signs; Toxicity =GI signs grade 1~4, at least once. Grouping 11130: No toxicity =No neutropenia or neutropenia grade 1.0~1.1; Toxicity =Neutropenia grade 2~3, at least once. Grouping 21130: No toxicity
=No anorexia or anorexia grade 1~2; Toxicity =Anorexia grade 3~4, at least once.
Grouping 31130: No toxicity =No vomiting or vomiting grade 1~2; Toxicity = Vomiting grade 3~4, at least once. Grouping 41130: No toxicity =No diarrhea or diarrhea grade 1~2; Toxicity = Diarrhea grade 3~4, at least once. Grouping 51130: No toxicity =No GI sign or GI signs grade 1~2; Toxicity = GI signs grade 3~4, at least once.
P values <0.05 were printed in bold type.
Grouping
Figure 10 cont’d
TTP: Time to tumor progression; OST: Overall survival time.
Grouping 11140: No toxicity =No neutropenia; Low-grade toxicity =Neutropenia grade 1.0~1.1, at least once; High-grade toxicity = Neutropenia grade 2~3, at least once.
Grouping 21140: No toxicity =No anorexia; Low-grade toxicity =Anorexia grade 1~2, at least once; High-grade toxicity =Anorexia grade 3~4, at least once. Grouping 31140:
No toxicity =No vomiting; Low-grade toxicity =Vomiting grade 1~2, at least once;
High-grade toxicity =Vomiting grade 3~4, at least once. Grouping 41140: No toxicity
=No diarrhea; Low-grade toxicity =Diarrhea grade 1~2, at least once; High-grade toxicity =Diarrhea grade 3~4, at least once. Grouping 51140: No toxicity =No GI signs;
Low-grade toxicity =GI signs grade 1~2, at least once; High-grade toxicity =GI signs grade 3~4, at least once.
P values <0.05 were printed in bold type.
Table 11
Intergroup P values for the 4 groupings dividing patients into three groups (ie., no toxicity group, low-grade toxicity group, and high-grade toxicity group) in Table 10.
TTP: Time to tumor progression; OST: Overall survival time.
Grouping 11140: No toxicity =No neutropenia; Low-grade toxicity =Neutropenia grade 1.0~1.1, at least once; High-grade toxicity = Neutropenia grade 2~3, at least once.
Grouping 31140: No toxicity =No vomiting; Low-grade toxicity =Vomiting grade 1~2, at least once; High-grade toxicity =Vomiting grade 3~4, at least once. Grouping 41140:
No toxicity =No diarrhea; Low-grade toxicity =Diarrhea grade 1~2, at least once;
High-grade toxicity =Diarrhea grade 3~4, at least once. Grouping 51140: No toxicity
=No GI signs; Low-grade toxicity =GI signs grade 1~2, at least once; High-grade toxicity =GI signs grade 3~4, at least once.
P values <0.05 were printed in bold type.
Table 12
Median TTP and OST and P values for the 3 frequency-adjusted groupings with P <0.05 and their non-frequency-adjusted counterparts.
Grouping 31113: No toxicity =No vomiting or vomiting ≦2 times; Toxicity =Vomiting grade 1~4, >2 times
Table 12 cont’d
High-grade toxicity =Vomiting grade 3~4, at least once. Grouping 31143: No toxicity =No vomiting or vomiting ≦2 times; Low-grade toxicity =Vomiting grade 1~2, >2 times; High-grade toxicity =Vomiting grade 3~4, >2 times. Grouping 41140: No toxicity =No diarrhea; Low-grade toxicity = Diarrhea grade
High-grade toxicity =Vomiting grade 3~4, at least once. Grouping 31143: No toxicity =No vomiting or vomiting ≦2 times; Low-grade toxicity =Vomiting grade 1~2, >2 times; High-grade toxicity =Vomiting grade 3~4, >2 times. Grouping 41140: No toxicity =No diarrhea; Low-grade toxicity = Diarrhea grade