Over the past three months, the Nasdaq Biotechnology index has tumbled around 9.5% and industrywide worries have pressured share prices of some companies that didn't deserve a market beatdown. Axsome Therapeutics (NASDAQ:AXSM), BridgeBio Pharma (NASDAQ:BBIO), and Kodiak Sciences (NASDAQ:KOD) have been top performers in 2019, and their recent pullbacks look like bargain opportunities that you'll want to take a closer look at.

Here's why these are the three best biotech stocks to buy in October.

Lab technicians in a cleanroom looking through microscopes.

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1. Axsome Therapeutics: On sale

A lengthy period without much news to report and a stock market scorned for biotech have pressured Axsome's shares around 42% lower over the past two weeks. Despite the recent tumble, shares of Axsome are still up 490% in 2019, and there are some big catalysts ahead.

Axsome's lead candidate, AXS-05, is a combination of dextromethorphan, the main ingredient in most over-the-counter cough suppressants, and bupropion, a decades-old antidepressant. These two drugs amplify each other and unintended mixing can be fatal. By carefully taking advantage of this interaction, though, Axsome effectively created a powerful new antidepressant. During a phase 2 testing Ascend study with major depressive disorder (MDD) patients, those treated with AXS-05 experienced improvements that blew past the group given bupropion on its own.

In addition to depression, bupropion's been used for smoking cessation, and it looks like AXS-05 could become a new option for people trying to break the habit. Axsome is also developing a migraine headache reliever, called AXS-07, that's in a pivotal study now and will probably deliver positive top-line results before the end of the year.

Even though an upcoming new drug application for AXS-05 looks like a slam dunk, the company's market cap at recent prices is a paltry $573 million. If AXS-05 is just half as successful as bupropion was, it could achieve 10-figure sales by 2024.

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2. BridgeBio Pharma: A new business model for biotech

The costs of drug development are still immense, but barriers to discovering new drug candidates have been plummeting. Once a small start-up has something that looks like a winner in a petri dish, it generally spends a lot of time and effort raising money to fund human studies. BridgeBio may have found a better way.

BridgeBio is actively pursuing over a dozen new drug programs, and each one is housed in a separate subsidiary, including AG10 from Eidos Therapeutics (NASDAQ:EIDX), which is in a phase 3 trial with transthyretin amyloidosis (ATTR) patients experiencing progressive heart damage.

Eidos Therapeutics has been developing its lead candidate for far less time than recently approved ATTR treatments from much larger companies. A successful launch for AG10 could give BridgeBio, which owns approximately two-thirds of Eidos, lots of capital to support its growing list of subsidiaries.

While BridgeBio focuses on raising money, its subsidiaries can keep their noses to the grindstone. That means investors can reasonably expect a flurry of new drug candidates entering clinical-stage trials over the next couple of years. At recent prices, BridgeBio sports a $2.4 billion market cap that could increase several times in the years ahead if another one of those subsidiaries gets as far down the development pathway as Eidos.

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3. Kodiak Sciences: Coming in heavy

How often do you see improvements to popular products that make them heavier? Kodiak Sciences is developing a new blindness prevention treatment that could be worth billions annually precisely because it's much heavier than similar drugs.

Invasive blood vessel growth at the back of the eyeballs is a common problem for people with diabetes, and the overall risk increases with age. Wet age-related macular degeneration (AMD) is the leading cause of vision loss among the elderly. Regeneron's Eylea is a drug that stops blood vessel proliferation or an anti-VEGF, and it's on pace to generate $5 billion in revenue this year.

Nobody likes sticking a needle in their eye at great expense every four to eight weeks. That's why in the real world, Eylea and other anti-VEGF drugs don't slow the rate of vision loss nearly as well as they could because there are usually several days -- or more if someone reschedules an appointment -- between injections when blood vessels are able to invade unprotected retinas.

Kodiak aims to alleviate between-dose vision loss with a really heavy anti-VEGF injection. Heavier drugs generally stick around longer, which is exactly what patients with progressive vision loss need in between each eyeball poke at the doctor's office.

Kodiak's lead candidate, KSI-301, is an anti-VEGF antibody attached to long chains of phosphorylcholine, the same biopolymer used in drug-eluting stents. The first 50 patients in a phase 1b study to reach a 12-week follow-up exam were losing vision to AMD, diabetic macular edema (DME), and retinal vein occlusion. After 12 weeks, patients treated with KSI-301 performed in line with results that led to Eylea's approval.

Kodiak's beginning a head-to-head trial with KSI-301 up against Eylea that could support a new drug application. At recent prices, Kodiak's a $563 million company that could break Regeneron's command of the enormous market for vision loss prevention. Further success in the clinic could send this biotech rocketing higher.

Overnight success?

Before the end of the year, Axsome will have several big catalysts, but it could be another couple of years before we know if Kodiak and BridgeBio will sink or swim. There's still a lot that can go wrong, but all three have what it takes to provide market-thumping gains over the long run.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.