Part XII · HER2-positive disease · Chapter 67

Biology and the treatment spine

One target, two decades of drugs against it, and the subtype that taught the field how to de-escalate.

1 · The biology this part assumes

This chapter carries no figures. It is the spine of the part, and the chapters after it are the evidence.

The defining fact of this subtype is an amplification. A segment of chromosome seventeen is present in many copies, the receptor it encodes is expressed far above normal, and the excess drives signalling without a ligand. HER family signaling sets out the receptor family and what the excess does to it.

Three properties follow.

The target is truncal. The amplification is an early event and it is carried by most cells in most of these tumours. Three things follow from that. Blockade keeps working across lines. The convention is to continue it while changing the partner. And this subtype has more active drugs against one target than any other in the disease. Temporal heterogeneity and clonal evolution is the chapter about which events are truncal and which are not.

The target is also heterogeneous, and the two statements are compatible. Truncal means most cells. It does not mean all cells, and the fraction that lacks the amplification is where resistance lives. HER2 heterogeneity is the chapter about that fraction, and it is the reason a conjugate can work where an antibody has failed.

The assay is a threshold on a continuum, and the threshold has moved. What counts as positive is a scoring convention over immunohistochemistry and in situ hybridisation. The category below it was not a category at all until a drug made it into one. Receptor assessment and the measurement problem is the measurement chapter and Expression and alteration-defined treatment across subtypes carries the argument about antigen thresholds as eligibility criteria, which is the same argument in its general form.

2 · What the subtype does to the treatment decision

Every regimen in this part is an answer to one of four questions. Naming them is the fastest way to read the chapters that follow.

What do you add to the antibody? The first question historically, and the one that built the adjuvant standard. Chemotherapy, then a second antibody, then a kinase inhibitor, each tested as an addition to a backbone that already worked.

What can you take away? This subtype invented modern de-escalation, for a structural reason. Response rates are high enough that a few hundred patients can separate two regimens, response arrives in months rather than years, and response carries more prognostic information here than anywhere else in the disease. That combination makes the neoadjuvant setting a usable instrument rather than a convenience, which The neoadjuvant setting as a scientific platform argues in general and Neoadjuvant therapy argues here.

What do you do when disease remains at surgery? Residual disease after adequate preoperative treatment identifies a group at high risk, and it is actionable rather than merely prognostic. This is the single most consequential use of the neoadjuvant platform in this book, and Adjuvant and post-neoadjuvant therapy carries it.

What do you give when the antibody stops working? The convention is that blockade continues and the partner changes. The convention is well founded and worth knowing what founds it, because the claim it does not support is that every continued regimen is equivalent. Metastatic therapy from first line onwards is the chapter about the sequence.

A fifth question applies to part of this population and is easy to miss. When the oestrogen receptor is also present, the two receptors converge on the cell cycle, and the tumour has a route out of each that it does not have out of both. That is why triple-positive disease sits in this part, and why it has two chapters of its own.

3 · The spine, from diagnosis to last line

Early disease starts with a decision about the operation, not the drug. Whether treatment is given before or after surgery changes what can be learned and what can be adapted. Deciding whether to treat before surgery frames the early decision and Neoadjuvant therapy argues the preoperative regimens.

The preoperative regimen is chosen for what it will reveal as well as for what it will do. A complete response allows de-escalation of what follows. Residual disease allows escalation. Both are handled in Adjuvant and post-neoadjuvant therapy, which is where the post-neoadjuvant decision lives.

Adjuvant treatment for a small node-negative tumour is a different problem. Those patients were largely excluded from the registration trials, so their regimen was built rather than inherited, and what is known about it comes from single-group and toxicity-endpoint studies. Adjuvant and post-neoadjuvant therapy states the margin a real de-escalation trial would need and why none exists.

Triple-positive early disease is where the two receptors first collide. Response to HER2-directed treatment is lower when the oestrogen receptor is present, which is a class finding rather than a property of one drug. Early triple-positive disease argues it and reads the chemotherapy-sparing strategies that follow from it.

Advanced disease has a settled first line and an unsettled order after it. When the target changes, and when to stop blocking it frames the metastatic decision and Metastatic therapy from first line onwards carries the line-by-line record, including the trials that have changed what first line means.

Triple-positive advanced disease asks whether the endocrine partner is maintenance or treatment. Metastatic triple-positive disease is the chapter, and its evidence is thinner than the equivalent evidence one part earlier.

The central nervous system is a separate compartment in this subtype more than in any other. It is not in this part. Central nervous system metastatic disease is the chapter, and the reason it is elsewhere is that the compartment does not sort by receptor.

4 · Where this part disagrees with itself

The de-escalation chapters and the escalation chapters read the same endpoint in opposite directions. A pathological complete response licenses giving less in Neoadjuvant therapy and predicts a good outcome in Adjuvant and post-neoadjuvant therapy. The trials that established the endpoint as prognostic are not the trials that established it as a licence to de-escalate, and the second claim is weaker than the first.

The duration trials disagree with each other, and the disagreement is not about trastuzumab. Adjuvant and post-neoadjuvant therapy carries four trials that set different non-inferiority margins on different scales, some anchored to a time point and some not. A reader who treats them as four attempts at one question will find them irreconcilable.

Triple-positive disease is filed here and argued partly elsewhere. Early triple-positive disease and Metastatic triple-positive disease sit in this part because their decisions are HER2 decisions. Several of the trials they turn on are endocrine trials, and the endocrine reasoning is in the previous part. The cross-references are load bearing rather than decorative.

The first-line record keeps moving, and the later-line record was built against an older first line. Metastatic therapy from first line onwards says this about itself. Every later-line trial in it enrolled patients who received a first-line regimen that may no longer be what a clinic gives, which limits what the sequence can be read to establish.